Yes, CAPTION is a Scrabble word!

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

Is it a valid word?

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

taking exception; especially a quibble based on a captious argument
translation of foreign dialogue of a movie or TV program; usually displayed at the bottom of the screen
brief description accompanying an illustration