n. A muscle, gland, or organ capable of responding to a stimulus, especially a nerve impulse.n. A nerve ending that carries impulses to a muscle, gland, or organ and activates muscle contraction or glandular secretion.n. Biochemistry A small molecule that when bound to an allosteric site of an enzyme causes either a decrease or an increase in the activity of the enzyme.n. Computer Science A device used to produce a desired change in an object in response to input.