Yes, SYNTHESIS is a Scrabble word!

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

Is it a valid word?

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

the process of producing a chemical compound (usually by the union of simpler chemical compounds)
the combination of ideas into a complex whole
reasoning from the general to the particular (or from cause to effect)

Also: synthetic thinking, deductive reasoning.