noun (n)

• being abnormally tolerant to and dependent on something that is psychologically or physically habit-forming (especially alcohol or narcotic drugs)(noun.state)

  Synonym:
  addiction, dependance, dependence, dependency
  

  source: wordnet30
• a general accommodation to unchanging environmental conditions(noun.act)

  source: wordnet30
• The act of habituating, or accustoming; the state of being habituated.(noun)

  source: webster1913