Yes, HORIZON is a Scrabble word!

Letter values in Scrabble US: H 4, O 1, R 1, I 1, Z 10, O 1, N 1.

Is it a valid word?

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

the line at which the sky and Earth appear to meet
the range of interest or activity that can be anticipated
a specific layer or stratum of soil or subsoil in a vertical cross section of land
the great circle on the celestial sphere whose plane passes through the sensible horizon and the center of the Earth

Also: apparent horizon, visible horizon, sensible horizon, celestial horizon.