1. The manufacture and discharge of specific substances into the external medium by cells in living organisms. (The substance secreted is also called the secretion.) Secretory cells are often specialized and organized in groups to form glands. The substances produced may be released directly into the blood (endocrine secretion; see endocrine gland) or through a duct (exocrine secretion; see exocrine gland). Secretions can be classified according to the manner of their discharge. Merocrine (eccrine) secretion occurs without the secretory cells sustaining any permanent change; in apocrine secretion the cells release a secretory vesicle incorporating part of the secretory cell membrane; and holocrine secretion involves the disruption of the entire cell to release its accumulated secretory vesicles. Substances destined for secretion are prepared and packaged into membranous vesicles by the Golgi apparatus inside the cell.

2. The process by which a substance is pumped out of a cell against a concentration gradient. Secretion has an important role in adjusting the composition of urine as it passes through the nephrons of the kidney.