From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]
From WordNet (r) 3.0 (2006) [wn]:
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}]