Yes, APPOSITION is a Scrabble word!

Letter values in Scrabble US: A 1, P 3, P 3, O 1, S 1, I 1, T 1, I 1, O 1, N 1.

Is it a valid word?

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

a grammatical relation between a word and a noun phrase that follows
(biology) growth in the thickness of a cell wall by the deposit of successive layers of material
the act of positioning close together (or side by side)