Yes, CERTIORARI is a Scrabble word!

Letter values in Scrabble US: C 3, E 1, R 1, T 1, I 1, O 1, R 1, A 1, R 1, I 1.

Is it a valid word?

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

a common law writ issued by a superior court to one of inferior jurisdiction demanding the record of a particular case

Also: writ of certiorari.