A biopsy is a medical procedure that involves taking a small sample of tissue so it can be examined under a microscope. Biopsies are used to identify abnormal cells. The result of a biopsy can often help healthcare professionals to diagnose a wide range of conditions.
Biopsies are commonly used to check whether a breast lump is benign (non-cancerous) or malignant (cancerous). After the tissue sample has been taken, the cells will be closely examined under a microscope to see whether or not they appear abnormal, and to check for any unusual cell activity. The cells may also be tested using various chemicals to see how they respond. The type of tests used will depend on the medical conditions that are being investigated.
Definition from NHS Choices: biopsy