Biopsy: The removal of a sample of tissue for purposes of diagnosis. (Many definitions of "biopsy" stipulate that the sample of tissue is removed for examination under a microscope. This may or may not be the case. The diagnosis may be achieved by other means such as by analysis of chromosomes or genes.)

A biopsy may be done, for example, because of concern about cancer. The physical exam, imaging, endoscopy, and laboratory tests may indicate that something is abnormal, but a biopsy may be the only sure way to know whether the problem is, in fact, cancer. In a biopsy, the doctor removes a sample of tissue from the abnormal area or may remove the whole tumor. A specialist trained to examine such tissues is called a pathologist. A pathologist examines the tissue under a microscope. If cancer is present, the pathologist can usually tell what kind of cancer it is and may be able to judge whether the cells are likely to grow slowly or quickly.