1. A muscle, gland, or organ capable of responding to a stimulus, especially a nerve impulse.

2. A nerve ending that carries impulses to a muscle, gland, or organ and activates muscle contraction or glandular secretion.

3. Biochemistry A small molecule or protein that alters biochemical processes in a cell, as by decreasing or increasing the activity of an enzyme.

4. Computers A device used to produce a desired change in an object in response to input.