Yes, AXIS is a Scrabble word!

Letter values in Scrabble US: A 1, X 8, I 1, S 1.

Is it a valid word?

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

a straight line through a body or figure that satisfies certain conditions
the main stem or central part about which plant organs or plant parts such as branches are arranged
in World War II the alliance of Germany and Italy in 1936 which later included Japan and other nations
a group of countries in special alliance
the 2nd cervical vertebra; serves as a pivot for turning the head
the center around which something rotates

Also: axis vertebra, axis of rotation.