signalling vs indicator

signalling and indicator both are nouns.

Word	Noun	Adjective	Verb	Adverb
signalling	Yes	No	No	No
indicator	Yes	No	No	No

As nouns, indicator is a hyponym of signalling; that is, indicator is a word with a more specific, narrower meaning than signalling:

signalling (noun)	indicator (noun)
any nonverbal action or gesture that encodes a message	a device for showing the operating condition of some system
	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
	(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