Amyloidosis: A disorder that results from the abnormal deposition of a particular protein, called amyloid, in various tissues of the body. Amyloid protein can be deposited in a localized area, and not be harmful, or in can cause serious changes in virtually any organ of the body.

Amyloidosis can occur as an isolated disease or as a result of another illness, including multiple myeloma, chronic infections (such as tuberculosis or osteomyelitis), or chronic inflammatory diseases (such as rheumatoid arthritis and ankylosing spondylitis). The protein that deposits in the brain of patients with Alzheimer's disease is a form of amyloid.