A section of an
organ that has been broken down, surrounded by an area of
inflammation and
infection. It takes the form of a rounded capsule, and walls itself off, keeping the rest of the body from
sepsis, but slowly getting the infected area worse. They are usually filled with
pus.
You can have an abscess in any organ, including extremeties. Most common cases are the lungs, I don't know how rare brain abscesses are.
Lung abscesses are usually caused by the inhalation of a foreign body like a peanut or tooth that jams itself into the lung tissue and inflames it. This happens in deeply unconscious patients or people who lack a gag reflex. It can also be caused by Tuberculosis as the body tries to contain the infection, or other airborne septic material.
Sometimes surgery is needed to remove it, sometimes just antibiotics and drainage (see postural drainage). In lung cases, a bronchoscope can be used to remove whatever object exists, unless the abscess is closer to the outside of the lung.