Yes, ALTITUDE is a Scrabble word!

Letter values in Scrabble US: A 1, L 1, T 1, I 1, T 1, U 1, D 2, E 1.

Is it a valid word?

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

elevation especially above sea level or above the earth's surface
the perpendicular distance from the base of a geometric figure to the opposite vertex (or side if parallel)
angular distance above the horizon (especially of a celestial object)