The Collaborative International Dictionary of English v.0.48:

Similarity \Sim`i*lar"i*ty\, n.; pl. -ties. [Cf. F.
   similarit['e].]
   The quality or state of being similar; likeness; resemblance;
   as, a similarity of features.
   [1913 Webster]

         Hardly is there a similarity detected between two or
         three facts, than men hasten to extend it to all. --Sir
                                                  W. Hamilton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

similarity
    n 1: the quality of being similar [ant: dissimilarity,
         unsimilarity]
    2: a Gestalt principle of organization holding that (other
       things being equal) parts of a stimulus field that are
       similar to each other tend to be perceived as belonging
       together as a unit [syn: similarity, law of similarity]