Webster's New World Medical Dictionary

A
In genetics, adenine, a member of the adenine-
thymine (A-T) base pair in DNA.
a-
Prefix indicating the absence or depletion of
something: for example, aphagia (not eating) or
aphonia (voiceless). The related prefix an- is usu-
ally used before a vowel, as in anemia (without
blood) and anoxia (without oxygen).
AA 1
Alcoholics Anonymous.
2
Amino acid.
AAAS
American Association for the Advancement
of Science, a professional organization that pub-
lishes the weekly journal 
Science.
AAFP
1
American Association of Family
Physicians, a professional organization for physi-
cians who treat both children and adults.
2
American Academy of Family Physicians, a profes-
sional organization for physicians who treat both
children and adults.
AAO 1
American Association of Ophthalmology, a
professional organization.
2
American Association
of Orthodontists, a professional organization.
3
American Academy of Otolaryngology, a professional
organization.
AAOS
American Academy of Orthopaedic
Surgeons, a professional organization. See also
orthopaedics.
AAP 1
American Academy of Pediatrics, a profes-
sional organization for physicians who treat 
infants, children, adolescents, and young adults.
2
American Academy of Pedodontics, a professional
organization.
3
American Academy of
Periodontology, a professional organization.
4
American Association of Pathologists, a professional
organization.
ab-
Prefix indicating from, away from, or off, as in
abduction (movement of a limb away from the mid-
line of the body) and abnormal (away from normal).
abdomen
The part of the body that contains all
the structures between the chest and the pelvis. The
abdomen, or belly, is anatomically separated from
the chest by the diaphragm, the powerful muscle
that spans the body cavity, just below the lungs. See
also 
abdominal cavity.