The Collaborative International Dictionary of English v.0.48:

Habituation \Ha*bit`u*a"tion\ (h[.a]*b[i^]t"[-u]*[=a]"sh[u^]n),
   n. [Cf. F. habituation.]
   The act of habituating, or accustoming; the state of being
   habituated.
   [1913 Webster]

WordNet (r) 3.0 (2006):

habituation
    n 1: being abnormally tolerant to and dependent on something
         that is psychologically or physically habit-forming
         (especially alcohol or narcotic drugs) [syn: addiction,
         dependence, dependance, dependency, habituation]
    2: a general accommodation to unchanging environmental
       conditions