Yes, CISTERN is a Scrabble word!

Letter values in Scrabble US: C 3, I 1, S 1, T 1, E 1, R 1, N 1.

Is it a valid word?

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

a sac or cavity containing fluid especially lymph or cerebrospinal fluid
a tank that holds the water used to flush a toilet
an artificial reservoir for storing liquids; especially an underground tank for storing rainwater

Also: water tank.