hormone

n. [ From Gr. "orma`ein to excite.] 1. (Physiological Chem.) A chemical substance formed in one organ and carried in the circulation to another organ on which it exerts a specific effect on cells at a distance from the producing cells; thus, pituitary hormones produced in the brain may have effects on cells in distant parts of the body.. [Webster 1913 Suppl. +PJC]

2. (Physiological Chem.) a chemical substance, whether natural or synthetic, that functions like a hormone in a living organism. Thus, synthetic steroid hormones may be more effective than their natural counterparts. [PJC]

3. (Bot.) A substance that controls growth rate or differentiation in plants; also called phytohormone. The most well-known are the auxins that stimulate growth at the growing tips of plants, and control root formation and the dropping of leaves; and the gibberellins, which are used in agriculture to promote plant growth. [PJC]