Yes, MECHANISM is a Scrabble word!

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

Is it a valid word?

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

the atomic process that occurs during a chemical reaction
the technical aspects of doing something
a natural object resembling a machine in structure or function
(philosophy) the philosophical theory that all phenomena can be explained in terms of physical or biological causes
device consisting of a piece of machinery; has moving parts that perform some function

Also: chemical mechanism.