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]

Moby Thesaurus II by Grady Ward, 1.0:

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