The Collaborative International Dictionary of English v.0.48:

Familiarity \Fa*mil`iar"i*ty\, n.; pl. Familiarities. [OE.
   familarite, F. familiarit['e]fr. L. faniliaritas. See
   Familiar.]
   1. The state of being familiar; intimate and frequent
      converse, or association; unconstrained intercourse;
      freedom from ceremony and constraint; intimacy; as, to
      live in remarkable familiarity.
      [1913 Webster]

   2. Anything said or done by one person to another
      unceremoniously and without constraint; esp., in the pl.,
      such actions and words as propriety and courtesy do not
      warrant; liberties.

   Syn: Acquaintance; fellowship; affability; intimacy. See
        Acquaintance.
        [1913 Webster]

WordNet (r) 3.0 (2006):

familiarity
    n 1: personal knowledge or information about someone or
         something [syn: acquaintance, familiarity,
         conversance, conversancy]
    2: usualness by virtue of being familiar or well known [ant:
       strangeness, unfamiliarity]
    3: close or warm friendship; "the absence of fences created a
       mysterious intimacy in which no one knew privacy" [syn:
       familiarity, intimacy, closeness]
    4: a casual manner [syn: casualness, familiarity]
    5: an act of undue intimacy [syn: familiarity, impropriety,
       indecorum, liberty]