Amyloid: Any of a number of complex proteins that are deposited in tissues and that share selected laboratory features such as a change in the fluorescence intensity of certain aromatic dyes like Congo Red.

The deposition of amyloid occurs in a number of diseases. In many of these, there is disagreement as to whether amyloid causes the disease or is simply a sign of the disease downstream from the cause. (In Alzheimer's disease, those who believe that the deposition of beta-amyloid protein kills neurons are called baptists.)

Amyloid may be deposited widely in the body, as in systemic amyloidosis. Or the deposition of amyloid may be organ-specific and limited, for instance, to the pancreas, as in type 2 diabetes, or the central nervous system, as in Alzheimer's disease, Parkinson's disease, Huntington disease, Creutzfeldt-Jakob disease and bovine spongiform encephalopathy (mad cow disease).

The term amyloid comes from amylo- (starch) + -oid (like) = like starch. This reflects the mistaken identification of the substance as starch based on crude staining techniques.