Yes, RELAXATION is a Scrabble word!

Letter values in Scrabble US: R 1, E 1, L 1, A 1, X 8, A 1, T 1, I 1, O 1, N 1.

Is it a valid word?

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

(physiology) the gradual lengthening of inactive muscle or muscle fibers
(physics) the exponential return of a system to equilibrium after a disturbance
a feeling of refreshing tranquility and an absence of tension or worry
an occurrence of control or strength weakening
freedom from activity (work or strain or responsibility)
a method of solving simultaneous equations by guessing a solution and then reducing the errors that result by successive approximations until all the errors are less than some specified amount
the act of making less strict

Also: relaxation behavior, relaxation method.

Not playable in Scrabble US: LIBERALISATION.