Yes, POLYSEMY is a Scrabble word!

Letter values in Scrabble US: P 3, O 1, L 1, Y 4, S 1, E 1, M 3, Y 4.

Is it a valid word?

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

the ambiguity of an individual word or phrase that can be used (in different contexts) to express two or more different meanings

Also: lexical ambiguity.