similarity

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}]
    

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]
    

65 Moby Thesaurus words for "similarity":
      accord, addition, adjunct, affairs, affiliation, affinity,
      alikeness, alliance, analogousness, analogy, approximation,
      assemblage, association, balance, bond, closeness, coincidence,
      collation, combination, commensurability, comparability,
      comparableness, comparativeness, comparison, connectedness,
      connection, contiguity, contrariety, correlation, dealings,
      deduction, disjunction, equivalence, filiation, homology,
      intercourse, interrelation, intimacy, junction, liaison, link,
      linkage, linking, mutual attraction, nearness, parallel,
      propinquity, proportion, proportionability, proximity, rapport,
      ratio, relatedness, relation, relations, relationship, resemblance,
      semblance, simile, similitude, sympathy, synonymity, tie, tie-in,
      union