Yes, CONVICTION is a Scrabble word!

Letter values in Scrabble US: C 3, O 1, N 1, V 4, I 1, C 3, T 1, I 1, O 1, N 1.

Is it a valid word?

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

an unshakable belief in something without need for proof or evidence
(criminal law) a final judgment of guilty in a criminal case and the punishment that is imposed

Also: strong belief, article of faith, judgment of conviction.