Yes, SYMPOSIUM is a Scrabble word!

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

Is it a valid word?

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

a meeting or conference for the public discussion of some topic especially one in which the participants form an audience and make presentations