Yes, POPULATION is a Scrabble word!

Letter values in Scrabble US: P 3, O 1, P 3, U 1, L 1, A 1, T 1, I 1, O 1, N 1.

Is it a valid word?

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

the people who inhabit a territory or state
a group of organisms of the same species inhabiting a given area
(statistics) the entire aggregation of items from which samples can be drawn
the number of inhabitants (either the total number or the number of a particular race or class) in a given place (country or city etc.)
the act of populating (causing to live in a place)