Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

Source : WordNet®

similarity
     n 1: the quality of being similar [ant: {dissimilarity}]
     2: a Getalt 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: {law of similarity}]