Yes, LANDMARK is a Scrabble word!

Letter values in Scrabble US: L 1, A 1, N 1, D 2, M 3, A 1, R 1, K 5.

Is it a valid word?

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

the position of a prominent or well-known object in a particular landscape
an event marking a unique or important historical change of course or one on which important developments depend
a mark showing the boundary of a piece of land
an anatomical structure used as a point of origin in locating other anatomical structures (as in surgery) or as point from which measurements can be taken

Also: turning point.