Yes, ASTIGMATISM is a Scrabble word!

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

Is it a valid word?

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

(ophthalmology) impaired eyesight resulting usually from irregular conformation of the cornea; common in nearsighted people
(optics) defect in an optical system in which light rays from a single point fail to converge in a single focal point