The Collaborative International Dictionary of English v.0.48:

Acclimatization \Ac*cli"ma*ti*za"tion\
   ([a^]k`kl[imac]"m[.a]*t[i^]*z[=a]"sh[u^]n), n.
   The act of acclimatizing; the process of inuring to a new
   climate, or the state of being so inured. --Darwin.
   [1913 Webster]

WordNet (r) 3.0 (2006):

acclimatization
    n 1: adaptation to a new climate (a new temperature or altitude
         or environment) [syn: acclimatization, acclimatisation,
         acclimation]