Yes, TRANSPOSITION is a Scrabble word!

Letter values in Scrabble US: T 1, R 1, A 1, N 1, S 1, P 3, O 1, S 1, I 1, T 1, I 1, O 1, N 1.

Is it a valid word?

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

any abnormal position of the organs of the body
an event in which one thing is substituted for another
(genetics) a kind of mutation in which a chromosomal segment is transfered to a new position on the same or another chromosome
(mathematics) the transfer of a quantity from one side of an equation to the other along with a change of sign
(electricity) a rearrangement of the relative positions of power lines in order to minimize the effects of mutual capacitance and inductance
the act of reversing the order or place of
(music) playing in a different key from the key intended; moving the pitch of a piece of music upwards or downwards

Not playable in Scrabble US: HETEROTAXY.