Yes, SIMILARITY is a Scrabble word!

Letter values in Scrabble US: S 1, I 1, M 3, I 1, L 1, A 1, R 1, I 1, T 1, Y 4.

Is it a valid word?

SIMILARITY is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the quality of being similar
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

Also: law of similarity.