Yes, RELATIVITY is a Scrabble word!

Letter values in Scrabble US: R 1, E 1, L 1, A 1, T 1, I 1, V 4, I 1, T 1, Y 4.

Is it a valid word?

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

(physics) the theory that space and time are relative concepts rather than absolute concepts
the quality of being relative and having significance only in relation to something else

Also: theory of relativity, relativity theory, einstein's theory of relativity.