Webster's New World Medical Dictionary

fibril
A small fiber, a fine thread.
fibrillation
In cardiology, an abnormal and
erratic twitching of the heart muscle.
fibrillation, atrial
An abnormal and irregular
heart rhythm in which electrical signals are gener-
ated chaotically throughout the upper chambers
(atria) of the heart. Many people with atrial fibrilla-
tion have no symptoms. Among those who do, the
most common symptom is an uncomfortable aware-
ness of the rapid and irregular heartbeat (palpita-
tions). Atrial fibrillation can promote the formation
of blood clots that travel from the heart to the brain,
resulting in stroke. Treatment of atrial fibrillation
involves risk-factor control, use of medications to
slow the heart rate and/or convert the heart to 
normal rhythm, and prevention of blood clots. Also
known as auricular fibrillation.
fibrillation, auricular
See 
fibrillation, atrial.
fibrillation, ventricular
An abnormal and
irregular heart rhythm in which there are rapid
uncoordinated fluttering contractions of the lower
chambers (ventricles) of the heart. Ventricular fib-
rillation disrupts the synchrony between the heart-
beat and the pulse beat. Ventricular fibrillation is
commonly associated with heart attacks and scar-
ring of the heart muscle from previous heart
attacks. Ventricular fibrillation is life threatening.
fibrin
The protein that is formed during normal
blood clotting and that is the essence of the clot.
fibrinogen
The protein from which fibrin is
formed in normal blood clotting.
fibroadenoma
The most common benign tumor
of the breast and the most common breast tumor in
women under 30 years of age. Fibroadenomas are
usually solitary and may increase in size during
pregnancy. The most common signs of fibroade-
noma are a painless, rubbery or firm, moveable
mass of the breast. The diagnosis of fibroadenoma
is made by biopsy. Treatment may involve surgical
removal. Fibroadenomas are rare in post-
menopausal women. Juvenile fibroadenomas are
rapidly growing, benign breast tumors that occur in
adolescent females.
fibroblast
A type of cell found in connective tis-
sue throughout the body that produces collagen and
other proteins found in the extracellular (between
cells) spaces.