Webster's New World Medical Dictionary

five-day fever
See 
trench fever.
flail chest
A condition that occurs when enough
ribs are broken (usually from a crush injury) to
compromise the rigidity of the chest wall. On inspi-
ration, the chest wall moves inward instead of out-
ward, and it does the opposite on expiration.
flat feet
Absence of an arch in the sole of the foot
that causes the foot to lie flat when the person is
standing. All babies have flat feet because their
arches are not yet built up (and their feet tend to be
plump). This condition may persist into adulthood,
or an arch may form as the child grows. Flat feet can
also be acquired, as in jobs that require a great deal
of walking and carrying of heavy objects. Also called
pes planus.
flatulence
The passing of gas from the intestinal
tract. Also commonly known as farting. See also
flatus.
flatus
Gas in the intestinal tract or gas passed
through the anus. Intestinal gas contains numerous
gases including oxygen, nitrogen, hydrogen, carbon
dioxide, hydrogen sulfide, ammonia, and methane.
The foul smell usually is caused by small traces of
gases such as hydrogen sulfide, ammonia, and
methane.
flavin-containing monooxygenase-3
FMO3.
flavivirus
One of a family of viruses transmitted
by mosquitos and ticks that cause some important
diseases, including dengue, yellow fever, tick-borne
encephalitis, and West Nile fever. Flaviviruses have
single-stranded RNA as their genetic material. 
flexion
The process of bending, or the state of
being bent. For example, flexion of the fingers
results in a clenched fist.
Flexner Report
A report, the full name of which
is “Medical Education in the United States and
Canada.” The 1910 report is named for its author,
professional educator Abraham Flexner, who
researched and wrote this report for the Carnegie
Foundation. At the time that the report was written,
many medical schools were proprietary schools
operated more for profit than for education. In their
stead, Flexner proposed medical schools in the
German tradition of strong biomedical sciences,
together with hands-on clinical training. The Flexner
Report caused many medical schools to close, and
most of the remaining schools were reformed to
conform to the Flexnerian model.
floaters
Spots in the field of vision due to
deposits of protein drifting about in the vitreous
humor (the clear jelly-like substance that fills the
middle of the eye). Also known as spots in front of
the eyes.
floating rib
The last two false ribs, which usually
have no ventral attachment to anchor them in front.
Also known as fluctuating or vertebral rib. See also
false rib.