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]

WordNet (r) 3.0 (2006):

hormone
    n 1: the secretion of an endocrine gland that is transmitted by
         the blood to the tissue on which it has a specific effect
         [syn: hormone, endocrine, internal secretion]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "hormone":
   Allen-Doisy hormone, adjuvant, adrenosterone, androgen,
   androsterone, autacoid, bile, carminative, chalone,
   cholecystokinin, chondrotrophic hormone, corticosterone,
   counterirritant, dehydrocorticosterone, digestive secretion,
   emmenagogue, endocrine, expectorant, gall, gastric juice, insulin,
   intestinal juice, maturative, mucus, pancreatic juice,
   progesterone, prostatic fluid, rheum, salivary secretion, semen,
   sperm, tears, thyroxin, vasodilator, vitamin