The Collaborative International Dictionary of English v.0.48:

Hormone \Hor"mone\ (h[^o]r"m[=o]n), 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]