Yes, CLIMAXES is a Scrabble word!

Letter values in Scrabble US: C 3, L 1, I 1, M 3, A 1, X 8, E 1, S 1.

Is it a valid word?

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

the highest point of anything conceived of as growing or developing or unfolding
the decisive moment in a novel or play
the moment of most intense pleasure in sexual intercourse
the most severe stage of a disease
arrangement of clauses in ascending order of forcefulness

Also: flood tide, sexual climax.