1. Any of various disorders of lipoprotein metabolism, usually characterized by abnormally high levels of cholesterol and certain lipoproteins in the blood.
2. An excess of lipoproteins (any of the lipid-protein, or fat-protein, complexes in which lipids or fats exist) in the blood, due to a disorder of lipoprotein metabolism.

It may be acquired or hereditary. Acquired forms accompany other disorders or may be caused by environmental factors such as diet.

There are a number of different hereditary forms, classified according to clinical features, enzymatic abnormalities, and serum lipoprotein patterns.