Yes, PARTIALS is a Scrabble word!

Letter values in Scrabble US: P 3, A 1, R 1, T 1, I 1, A 1, L 1, S 1.

Is it a valid word?

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

the derivative of a function of two or more variables with respect to a single variable while the other variables are considered to be constant
a harmonic with a frequency that is a multiple of the fundamental frequency

Also: partial derivative, partial tone.