Yes, OPACITY is a Scrabble word!

Letter values in Scrabble US: O 1, P 3, A 1, C 3, I 1, T 1, Y 4.

Is it a valid word?

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

the phenomenon of not permitting the passage of electromagnetic radiation
incomprehensibility resulting from obscurity of meaning
the quality of being opaque to a degree; the degree to which something reduces the passage of light