The Collaborative International Dictionary of English v.0.48:

Informality \In`for*mal"i*ty\, n.; pl. Informalities.
   1. The state of being informal; lack of regular, prescribed,
      or customary form; as, the informality of legal
      proceedings.
      [1913 Webster]

   2. An informal, unconventional, or unofficial act or
      proceeding; something which is not in proper or prescribed
      form or does not conform to the established rule.
      [1913 Webster]

WordNet (r) 3.0 (2006):

informality
    n 1: a manner that does not take forms and ceremonies seriously
         [ant: formality, formalness]
    2: freedom from constraint or embarrassment; "I am never at ease
       with strangers" [syn: ease, informality]