The Collaborative International Dictionary of English v.0.48:

homeostasis \homeostasis\ n.
   The ability and tendency of certain systems to maintain a
   relatively constant internal state in spite of changes in
   external conditions; this ability is achieved by the presence
   of feedback mechanisms which can adjust the state of the
   system to compensate for changes in the state caused by the
   external environment. It is exemplified in homeothermal
   biological systems, such as animals which maintain relatively
   constant blood temperature and composition in spite of
   variations in external temperature or the composition of the
   food ingested.
   [PJC]

WordNet (r) 3.0 (2006):

homeostasis
    n 1: (physiology) metabolic equilibrium actively maintained by
         several complex biological mechanisms that operate via the
         autonomic nervous system to offset disrupting changes

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "homeostasis":
   aplomb, balance, balanced personality, constancy, cool,
   equilibrium, fastness, firmness, imperturbability, invariability,
   nerve, reliability, rootedness, sang-froid, secureness, security,
   solidity, soundness, stability, stable equilibrium, stable state,
   steadfastness, steadiness, steady nerves, steady state,
   substantiality, undeflectability, uniformity, unshakable nerves,
   unshakableness