Yes, QUADRANT is a Scrabble word!

Letter values in Scrabble US: Q 10, U 1, A 1, D 2, R 1, A 1, N 1, T 1.

Is it a valid word?

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

a quarter of the circumference of a circle
any of the four areas into which a plane is divided by two orthogonal coordinate axes
the area enclosed by two perpendicular radii of a circle
a measuring instrument for measuring altitude of heavenly bodies

Also: quarter-circle.