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]

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "familiarity":
   Bohemianism, acquaintance, affability, affinity, amiability,
   assumption, awareness, boldness, casualness, chumminess, civility,
   closeness, clubbability, clubbishness, clubbism, cognition,
   cognizance, common touch, communicativeness, companionability,
   compatibility, comprehension, congeniality, cordiality, corpus,
   courtesy, data, datum, ease, easiness, easygoingness, experience,
   expertise, facts, factual base, fellowship, folksiness,
   fondness for society, friendliness, geniality, graciousness, grasp,
   gregariousness, homeliness, homeyness, hospitality, hubris,
   impertinence, imposition, impropriety, impudence, informality,
   informalness, information, inseparableness, insolence,
   intelligence, intimacy, intimate acquaintance, inwardness,
   irregularity, ken, know-how, knowing, knowledge, lawlessness,
   liberties, liberty abused, license, licentiousness, looseness,
   mateyness, naturalness, nearness, offhandedness, openness,
   palliness, plainness, practical knowledge, presumption,
   presumptuousness, private knowledge, privity, ratio cognoscendi,
   relaxedness, self-knowledge, simplicity, sociability, sociableness,
   social grace, sociality, special affinity, technic, technics,
   technique, unaffectedness, unceremoniousness, unconstrainedness,
   unconstraint, unconventionality, understanding, undue liberty,
   unpretentiousness, urbanity