second nature

from WordNet (r) 3.0 (2006)

second nature
    n 1: acquired behavior that is practiced so long it seems innate

from Moby Thesaurus II by Grady Ward, 1.0

20 Moby Thesaurus words for "second nature":
      automatism, bad habit, characteristic, creature of habit, custom,
      force of habit, habit, habit pattern, habitude, pattern,
      peculiarity, practice, praxis, stereotype, stereotyped behavior,
      trick, usage, use, way, wont