in biology, enclosed sac within body tissues, having a distinct membrane and generally containing a liquid material. In the life cycle of certain parasitic worms, a cyst develops around the larval form within the muscle tissue of the host animal.

Although the majority of cysts are benign, several varieties may be malignant or precancerous. Benign cysts often require removal because they interfere with surrounding organs. Cysts form from a proliferation of epithelium, the tissue making up the skin and the linings of the blood vessels and body cavities, and may become detached from surrounding structures so that they move freely. The material inside can consist of natural body secretions, abnormal products from the breakdown of natural secretions and structural proteins, or, in case of infection, bacteria, larval parasites, and microbial products.

Several organs, including the kidney, liver, and breast (mammary gland), are particularly susceptible to cyst formation and may become filled with numerous cysts of various sizes. In some cases, these cystic diseases are themselves dangerous, or they may obscure more serious, underlying diseases.