Yes, DIFFUSION is a Scrabble word!

Letter values in Scrabble US: D 2, I 1, F 4, F 4, U 1, S 1, I 1, O 1, N 1.

Is it a valid word?

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

(physics) the process in which there is movement of a substance from an area of high concentration of that substance to an area of lower concentration
the spread of social institutions (and myths and skills) from one society to another
the property of being diffused or dispersed
the act of dispersing or diffusing something