Secretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product. Many cells contain this such as glucoma cells.
Tissues in humans that produce secretions include the gastrointestinal tract which secretes digestive enzymes and gastric acid, the lung which secretes surfactants, and sebaceous glands which secrete sebum to lubricate the skin and hair. Meibomian glands in the eyelid secrete sebum to lubricate and protect the eye.