Yes, INDICATOR is a Scrabble word!

Letter values in Scrabble US: I 1, N 1, D 2, I 1, C 3, A 1, T 1, O 1, R 1.

Is it a valid word?

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

a number or ratio (a value on a scale of measurement) derived from a series of observed facts; can reveal relative changes as a function of time
a signal for attracting attention
a device for showing the operating condition of some system
(chemistry) a substance that changes color to indicate the presence of some ion or substance; can be used to indicate the completion of a chemical reaction or (in medicine) to test for a particular reaction

Also: index number.