Yes, DOMAINS is a Scrabble word!

Letter values in Scrabble US: D 2, O 1, M 3, A 1, I 1, N 1, S 1.

Is it a valid word?

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

a particular environment or walk of life
territory over which rule or control is exercised
(mathematics) the set of values of the independent variable for which a function is defined
people in general; especially a distinctive group of people with some shared interest
the content of a particular field of knowledge

Also: domain of a function, knowledge domain, knowledge base.