Yes, ETERNITY is a Scrabble word!

Letter values in Scrabble US: E 1, T 1, E 1, R 1, N 1, I 1, T 1, Y 4.

Is it a valid word?

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

time without end
a state of eternal existence believed in some religions to characterize the afterlife
a seemingly endless time interval (waiting)

Also: timeless existence.