Yes, PERIOD is a Scrabble word!

Letter values in Scrabble US: P 3, E 1, R 1, I 1, O 1, D 2.

Is it a valid word?

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

an amount of time
the interval taken to complete one cycle of a regularly repeating phenomenon
(ice hockey) one of three divisions into which play is divided in hockey games
a unit of geological time during which a system of rocks formed
the end or completion of something
the monthly discharge of blood from the uterus of nonpregnant women from puberty to menopause
a punctuation mark (.) placed at the end of a declarative sentence to indicate a full stop or after abbreviations

Also: time period, period of time, geological period, full stop, full point.