merit pay vs earnings

merit pay and earnings both are nouns.

Word	Noun	Adjective	Verb	Adverb
merit pay	Yes	No	No	No
earnings	Yes	No	No	No

As nouns, earnings is a hypernym of merit pay; that is, earnings is a word with a broader meaning than merit pay:

merit pay: extra pay awarded to an employee on the basis of merit (especially to school teachers)
earnings: something that remunerates

Other hypernyms of merit pay include pay, remuneration, salary, wage.

merit pay (noun)	earnings (noun)
extra pay awarded to an employee on the basis of merit (especially to school teachers)	something that remunerates
	the excess of revenues over outlays in a given period of time (including depreciation and other non-cash expenses)