Yes, CONCLUSION is a Scrabble word!

Letter values in Scrabble US: C 3, O 1, N 1, C 3, L 1, U 1, S 1, I 1, O 1, N 1.

Is it a valid word?

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

a position or opinion or judgment reached after consideration
an intuitive assumption
the temporal end; the concluding time
event whose occurrence ends something
the proposition arrived at by logical reasoning (such as the proposition that must follow from the major and minor premises of a syllogism)
the act of ending something
a final settlement
the last section of a communication
the act of making up your mind about something

Also: stopping point.