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Medical Dictionary


  • omega, the 24th and last letter of the Greek alphabet
  • ohm
  • oxygen
  • orotidine
  • pint — used in writing prescriptions (from the Latin octarius)
  • opening (in formulas for electrical reactions)
  • one of the four ABO blood groups characterized by the absence of antigens designated by the letters A and B and by the presence of antibodies against these antigens.
  • abbr: ohne Hauch (without a film), a designation for antigens that occur in the bacterial cell, in contrast to those in the flagella; in specific antibodies for such somatic antigens; in the agglutinative reaction between somatic antigen and its antibody.
  • abbr: orth- or ortho-
  • egg : ovum, e.g., oocyte
O antigen
n antigen that occurs in the body of a gram-negative bacterial cell, aka somatic antigen

O-acetylcarnitine (as-e-til-kar′ni-ten)
The acetyl derivative of carnitine formed by carnitine acetyltransferase. Facilitates acetyl transport into the mitochondria and is an important fuel source for sperm.

o-aminobenzoic acid (a-me′no-ben-zo′ik)
SYN: anthranilic acid.

o-chlorobenzalmalononitrile (or′tho-klor′o-ben-zal-ma-lon′o-ni-tril)
A strong lacrimator used in riot control.

An antiseptic liquid, used in the treatment of lupus.

o-diphenolase (di-fen′o-las)
SYN: catechol oxidase.

Symbol for oxygen-15.

Symbol for oxygen-16.

Symbol for oxygen-17.

Symbol for oxygen-18.

Abbreviation for occipitoanterior position.

oak apple
SYN: nutgall.

oari-, oario-
Obsolete term for an ovary. See oo-, oophor-, ovario-. [G. oarion, a small egg, dim. of oon, egg]

oath (oth)
A solemn affirmation or attestation.

Abbreviation for obstetrics.

James, Irish surgeon, 1786–1862. See O. sphincter.

obeliac (o-be′le-ak)
Relating to the obelion.

obeliad (o-be′le-ad)
Toward the obelion.

obelion (o-be′le-on)
A craniometric point on the sagittal suture between the parietal foramina near the lambdoid suture. [G. obelos, a spit]

Friedrich, Austrian physician, 1861–1925. See O. test.

Otto H.F., German physician, 1843–1873. See O. spirillum.

Heinrich, Austrian neurologist, 1847–1922. See O.-Redlich line, O.-Redlich zone.

obese (o-bes′)
Excessively fat. SYN: corpulent. [L. obesus, fat, partic. adj., fr. ob-edo, pp. -esus, to eat away, devour]

obesity (o-be′si-te)
An excess of subcutaneous fat in proportion to lean body mass. Excess fat accumulation is associated with increase in the size (hypertrophy) as well as the number (hyperplasia) of adipose tissue cells. O. is variously defined in terms of absolute weight, weight-height ratio, distribution of subcutaneous fat, and societal and esthetic norms. Measures of weight in proportion to height include relative weight (RW, body weight divided by median desirable weight for a person of the same height and medium frame according to actuarial tables), body mass index (BMI, kg/m2) and ponderal index (kg/m3). These do not differentiate between excess adiposity and increased lean body mass. In contrast, subscapular and triceps skinfold measurements and determination of the waist-to-hip ratio help define the regional deposition of fat and differentiate the more medically significant central o. from peripheral o. in adults. No single cause can explain all cases of o.. Ultimately it results from an imbalance between energy intake and energy expenditure. While faulty eating habits related to failure of normal satiety feedback mechanisms may be responsible for some cases, many obese persons neither consume more calories nor eat different proportions of foodstuffs than nonobese persons. Contrary to popular belief, o. is not caused by disorders of pituitary, thyroid, or adrenal gland metabolism. However, it is often associated with hyperinsulinism and relative insulin resistance. Studies of obese twins strongly suggest the presence of genetic influences on resting metabolic rate, feeding behavior, changes in energy expenditures in response to overfeeding, lipoprotein lipase activity, and basal rate of lipolysis. Environmental factors associated with o. include socioeconomic status, race, region of residence, season, urban living, and being part of a smaller family. The prevalence of o. is greater when weight is measured during winter rather than summer. O. is much commoner in the northeastern and midwestern U.S. than in the south and west, a phenomenon independent of race, population density, and season. SYN: adiposity (1) , corpulence, corpulency. [L. obesus, pp. of obedo, to eat up, + -ity] O. is a major public health problem and the leading nutritional disorder in the U.S. A widely accepted definition of o. is body weight that is 20% or more in excess of ideal weight-for-height according to actuarial tables. By this definition, 34% of adults in the U.S. are obese, and there is evidence that the prevalence of o. is increasing in both children and adults. The National Institutes of Health have defined o. as a BMI of 30 kg/m2 or more, and overweight as a BMI between 25–30 kg/m2. By these criteria, 55% of adults are either overweight or obese. O. is an independent risk factor for hypertension, hypercholesterolemia, type 2 diabetes mellitus, myocardial infarction, certain malignancies (cancer of the colon, rectum, and prostate in men and of the breast, cervix, endometrium, and ovary in women), obstructive sleep apnea, hypoventilation syndrome, osteoarthritis and other orthopedic disorders, infertility, lower extremity venous stasis disease, gastroesophageal reflux disease, and urinary stress incontinence. Lesser degrees of o. can constitute a significant health hazard in the presence of diabetes mellitus, hypertension, heart disease, or their associated risk factors. Body fat distribution in central (abdominal or male pattern, with an increased waist-to-hip ratio) versus peripheral (gluteal or female pattern) adipose tissue deposit is associated with higher risks of many of these disorders. Obese persons are more liable to injury, more difficult to examine by palpation and imaging techniques, and more likely to have unsuccessful outcomes and complications from surgical operations. Not least among the adverse effects of o. are social stigmatization, poor self-image, and psychological stress. Weight reduction is associated with improvement in most of the health risks of o.. All treatments for o. (other than cosmetic surgical procedures in which subcutaneous fat is mechanically removed) require creation of an energy deficit by reducing caloric intake, increasing physical exercise, or both. Basic weight-reduction programs involve consumption of a restricted-calorie, low-fat diet and performance of at least 30 minutes of endurance-type physical activity of at least moderate intensity on most and preferably all days of the week. Behavior modification therapy, hypnosis, anorexiant drugs, and surgical procedures to reduce gastric capacity or intestinal absorption of nutrients are useful in selected cases, but the emphasis should be on establishing permanent changes in lifestyle. Weight reduction is not recommended during pregnancy or in patients with osteoporosis, cholelithiasis, severe mental illness including anorexia nervosa, or terminal illness. android o. central o. (apple shape) with fat excess primarily in abdominal wall and visceral mesentery; associated with glucose intolerance, diabetes, decreased sex hormone–binding globulin, increased levels of free testosterone, and increased cardiovascular risk. gynecoid o. o. with fat excess mainly in the femoral-gluteal region (pear shape). hypothalamic o. o. caused by disease of the hypothalamus. hypothalamic o. with hypogonadism SYN: adiposogenital dystrophy. morbid o. o. sufficient to prevent normal activity or physiologic function, or to cause the onset of a pathologic condition. simple o. o. resulting when caloric intake exceeds energy expenditure.

obex (o′beks) [TA]
The point on the midline of the dorsal surface of the medulla oblongata that marks the caudal angle of the rhomboid fossa or fourth ventricle. It corresponds to a small, transverse medullary fold overhanging the calamus scriptorius. [L. barrier]

obfuscation (ob-fus-ka′shun)
1. A rendering dark or obscure. 2. A deliberate attempt to confuse or to prevent understanding. [L. ob-fusco, pp. -atus, to darken, fr. fuscus, dark, tawny]

Abbreviation for obstetrics and gynecology.

obidoxime chloride (ob′e-dok-sem)
A cholinesterase reactivator much like 2-PAM.

object (ob′jekt)
1. Anything to which thought or action is directed. 2. In psychoanalysis, that through which an instinct can achieve its aim. 3. In psychoanalysis, often used synonymously with person. good o. in psychoanalysis, the good or supporting aspects of an important person in the patient's life, especially of a parent or parent-surrogate. sex o. a person toward whom another is sexually attracted; a term most used by a female to indicate that a male narrowly views her as a vehicle for sex while completely disregarding the rest of her persona. test o. 1. an o. having very fine surface markings, mounted on a slide, used to determine the defining power of the objective lens of a microscope; 2. the target in measurement of the visual field. transitional o. an o. used by many children as a substitute for a parent who is absent (usually temporarily) to help them deal with separation; typically, a blanket or stuffed toy.

object choice
In psychoanalysis, the object (usually a person) upon which psychic energy is centered.

objective (ob-jek′tiv)
1. The lens or lenses in the object end of the body tube of a microscope, by means of which the rays coming from the object examined are brought to a focus. SYN: object glass. 2. Viewing events or phenomena as they exist in the external world, impersonally, or in an unprejudiced way; open to observation by oneself and by others. Cf.:subjective. [L. ob- jicio, pp. -jectus, to throw before] achromatic o. an o. that is corrected for two colors chromatically, and one color spherically. apochromatic o. an o. in which chromatic aberration is corrected for three colors and spherical aberration is corrected for two. immersion o. a high power o. used with a drop of oil between the lens and the specimen on the slide, allowing a greater numerical aperture; similar lenses are available for use with water as the immersing liquid.

objective assessment data
Those facts that are observable and measurable by the nurse.

obligate (ob′li-gat)
Without an alternative system or pathway. [L. ob-ligo, pp. -atus, to bind to]

oblique (ob-lek′)
Slanting; deviating from the perpendicular, horizontal, sagittal, or coronal plane of the body. In radiography, a projection that is neither frontal nor lateral. [L. obliquus]

obliquity (ob-lik′wi-te)
SYN: asynclitism. Litzmann o. inclination of the fetal head so that the biparietal diameter is oblique in relation to the plane of the pelvic brim, the posterior parietal bone presenting to the parturient canal. SYN: posterior asynclitism. Nägele o. inclination of the fetal head in cases of flat pelvis, so that the biparietal diameter is oblique in relation to the plane of the pelvic brim, the anterior parietal bone presenting to the parturient canal. SYN: anterior asynclitism.

obliquus (ob-li′kwus)
Denoting a structure having an oblique course or direction; a name given, with further qualification, to several muscles. See muscle. [L. slanting, oblique]

obliteration (ob-lit-er-a′shun)
Blotting out, especially by filling of a natural space or lumen by fibrosis or inflammation. In radiology, disappearance of the contour of an organ when the adjacent tissue has the same x-ray absorption. See silhouette sign of Felson. [L. oblittero, to blot out] osteoplastic o. of the frontal sinus operation to remove the diseased contents, including the mucous membrane, of the frontal sinus and to obliterate the sinus with a free fat graft without altering the external contour of the sinus.

oblongata (ob-long-gah′ta)
SYN: medulla o.. [L. fem. of oblongatus, from oblongus, rather long]

obnubilation (ab-noo′bil-a′shun)
A clouded mental state. [L. ob-nubilo, to becloud, obscure, fr. nubes, cloud]

Abbreviation for organic brain syndrome.

observer (ob-zer′ver)
One who perceives, notices, or watches; in behavioral research with humans, the investigator or his/her surrogate. [L. observo, to watch] nonparticipant o. an investigator who studies a group of subjects engaged in certain activities but does not directly participate in these activities, presumably being able to study them more objectively. participant o. an investigator who while studying the activities of a group of subjects also participates in their activities, presumably being able to gain more detailed, relevant information but with less objectivity.

obsession (ob-sesh′un)
A recurrent and persistent idea, thought, or impulse to carry out an act that is ego-dystonic, that is experienced as senseless or repugnant, and that the individual cannot voluntarily suppress. [L. obsideo, pp. -sessus, to besiege, fr. sedeo, to sit] impulsive o. an o. accompanied by action, sometimes becoming a mania. inhibitory o. an o. involving an impediment to action, usually representing a phobia.

Having a tendency to perform certain repetitive acts or ritualistic behavior to relieve anxiety, as in o. neurosis ( e.g., a compulsive, ritualistic need to wash one's hands many dozens of times per day).

obsolescence (ob-so-les′ens)
Falling into disuse; denoting the abolition of a function. [L. obsolesco, to grow out of use]

obstetric, obstetrical (ob-stet′rik, -ri-kal)
Relating to obstetrics.

obstetrician (ob-ste-tri-sh′un)
A physician specializing in the medical care of women during pregnancy and childbirth. [see obstetrics]

obstetrics (OB) (ob-stet′riks)
The specialty of medicine concerned with the care of women during pregnancy, parturition, and the puerperium. SYN: tocology. [L. obstetrix, a midwife, fr. ob-sto, to stand before, denoting the position formerly taken by the midwife]

obstinate (ob′sti-nat)
1. Firmly adhering to one's own purpose or opinion, even when wrong; not yielding to argument, persuasion, or entreaty. SYN: intractable (2) , refractory (2) . 2. SYN: refractory (1) . [L. obstinatus, determined]

obstipation (ob-sti-pa′shun)
Intestinal obstruction; severe constipation. [L. ob, against, + stipo, pp. -atus, to crowd]

obstruction (ob-struk′shun)
Blockage, clogging, or impeded flow, e.g., by occlusion or stenosis. [L. obstructio] closed loop o. o. of a segment of intestine either rotated on a fixed point (volvulus) or herniated through a fibrous opening (as under an adhesion or into a hernia); frequently associated with impaired perfusion ultimately resulting in gangrene. ureteropelvic junction o. an impediment to drainage of urine from kidney usually due to partial or intermittent blockage of renal collecting system at the junction of renal pelvis and ureter. ureterovesical o. o. of the lower ureter at its entrance into the bladder.

obstruent (ob′stroo-ent)
1. Rarely used term for obstructing or clogging. 2. Rarely used term for an agent that obstructs or prevents a normal discharge, especially a discharge from the bowels. [L. obstruo, to build against, obstruct]

obtund (ob-tund′)
To dull or blunt, especially to blunt sensation or deaden pain. [L. ob-tundo, pp. -tusus, to beat against, blunt]

obturation (ob-too-ra′shun)
Obstruction or occlusion. [see obturator] intermittent self-o. passage of a blunt object in a lumen or meatus to occlude it or to dilate it.

obturator (ob′too-ra-tor)
1. Any structure that occludes an opening. 2. Denoting the o. foramen, the o. membrane, or any of several parts in relation to this foramen. 3. A prosthesis used to close an opening of the hard palate, usually a cleft palate. 4. The stylus or removable plug used during the insertion of many tubular instruments. [L. obturo, pp. -atus, to occlude or stop up]

obtuse (ob-toos′)
1. Dull in intellect; of slow understanding. 2. Blunt; not acute. [see obtund]

obtusion (ob-too′zhun)
1. Dullness of sensibility. 2. A dulling or deadening of sensibility.

Occam's razor
The principle of scientific parsimony. William of Occam (14th century) stated it thus: “The assumptions introduced to explain a thing must not be multiplied beyond necessity.”

occipital (ok-sip′i-tal)
Relating to the occiput; referring to the o. bone or to the back of the head. SYN: occipitalis.

occipitalis (ok′sip-i-ta′lis)
SYN: occipital. [L.]

occipitalization (ok′sip′i-tal-i-za′shun)
Bony ankylosis between the atlas and occipital bone.

The occiput, occipital structures. [L. occiput]

occipitoatloid (ok-sip′i-to-at′loyd)
Relating to the occipital bone and the atlas; denoting the articulation between the two bones.

occipitoaxial, occipitoaxoid (ok-sip′i-to-ak′se-al, -ak′soyd)
Relating to the occipital bone and the axis, or epistropheus.

occipitobregmatic (ok-sip′i-to-breg-mat′ik)
Relating to the occiput and the bregma; denoting a measurement in craniometry.

occipitofacial (ok-sip′i-to-fa′shal)
Relating to the occiput and the face.

occipitofrontal (ok-sip′i-to-frun′tal)
1. Relating to the occiput and the forehead. 2. Relating to the occipital and frontal lobe of the cerebral cortex and association pathways that interconnect these regions.


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