Yes, VARIABLES is a Scrabble word!

Letter values in Scrabble US: V 4, A 1, R 1, I 1, A 1, B 3, L 1, E 1, S 1.

Is it a valid word?

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

something that is likely to vary; something that is subject to variation
a quantity that can assume any of a set of values
a star that varies noticeably in brightness
a symbol (like x or y) that is used in mathematical or logical expressions to represent a variable quantity

Also: variable quantity, variable star.