Yes, TRAPEZIUM is a Scrabble word!

Letter values in Scrabble US: T 1, R 1, A 1, P 3, E 1, Z 10, I 1, U 1, M 3.

Is it a valid word?

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

a quadrilateral with no parallel sides
a multiple star in the constellation of Orion
the wrist bone on the thumb side of the hand that articulates with the 1st and 2nd metacarpals

Also: trapezium bone, os trapezium.