Yes, IMAGINARIES is a Scrabble word!

Letter values in Scrabble US: I 1, M 3, A 1, G 2, I 1, N 1, A 1, R 1, I 1, E 1, S 1.

Is it a valid word?

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

(mathematics) a number of the form a+bi where a and b are real numbers and i is the square root of -1

Also: complex number, complex quantity, imaginary number.