Yes, DIAMETERS is a Scrabble word!

Letter values in Scrabble US: D 2, I 1, A 1, M 3, E 1, T 1, E 1, R 1, S 1.

Is it a valid word?

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

the length of a straight line passing through the center of a circle and connecting two points on the circumference
a straight line connecting the center of a circle with two points on its perimeter (or the center of a sphere with two points on its surface)

Not playable in Scrabble US: DIAM.