Webster's New World Medical Dictionary

Reiter syndrome
See 
reactive arthritis.
rejection
In transplantation biology, when the
body’s immune system attacks transplanted cells,
tissues, or organs. For example, a transplanted kid-
ney may be rejected.
relapse
The return of signs and symptoms of a
disease after a remission.
relaxant
Something that relaxes, relieves, or
reduces tension. For example, a muscle relaxant is
often administered during abdominal surgery to
relax the diaphragm and keep it from moving dur-
ing the surgery.
relaxin
A hormone that is produced during preg-
nancy that facilitates the birth process by causing a
softening and lengthening of the cervix and the
pubic symphysis (the place where the pubic bones
come together). Relaxin also inhibits contractions
of the uterus and may play a role in determining the
timing of delivery. 
release, carpal tunnel
See 
carpal tunnel
release.
rem
In radiation, an international unit of X-ray or
gamma-ray radiation adjusted for the atomic
makeup of the human body.
REM
Rapid eye movement of the eyes during
sleep. During REM sleep, the eyeballs appear to flick
around under the closed eyelids. See 
REM sleep.
REM sleep
The portion of sleep during which
rapid eye movements (REMs) occur. Dreams occur
during REM sleep, and people typically have three to
five periods of REM sleep per night. These periods
occur at intervals of 1 to 2 hours and can vary in
length from 5 minutes to over an hour. REM sleep is
also characterized by rapid, low-voltage brain waves
that are detectable on an electroencephalogram
(EEG) recording; irregular breathing and heart rate;
and involuntary muscle jerks. See also 
NREM sleep.