(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..
(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.
(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.