Similarity, n.; pl. -ties.
Etymology: [Cf. F. similarité.]
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.
Shades of meaning
Fernald has no entry of its own for similarity; he lists it under analogy, approximation.
Where he writes about it:
Resemblance and similarity are external or superficial, and may involve no deeper relation; as, the resemblance of a cloud to a distant mountain. — from analogy
Resemblance and similarity may be but superficial and apparent; approximation is real. — from approximation