1. As defined by Sherrington, a peripheral tissue that receives nerve impulses and reacts by muscular contractioon, glandular secretion, or electric discharge (from an electric organ, as in the case of certain bony fishes such as the electric eel). 2. A small metabolic molecule that, by combining with a repressor gene, depresses the activity of an operon. 3. A small molecule that binds to a protein or other macromolecule and, in so doing, alters the activity of that macromolecule. 4. An individual or a substance, technique, procedure, or person causing an effect.