Yes, NIHILISM is a Scrabble word!

Letter values in Scrabble US: N 1, I 1, H 4, I 1, L 1, I 1, S 1, M 3.

Is it a valid word?

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

a revolutionary doctrine that advocates destruction of the social system for its own sake
the delusion that things (or everything, including the self) do not exist; a sense that everything is unreal
complete denial of all established authority and institutions

Also: nihilistic delusion.