earnings vs merit pay

earnings and merit pay both are nouns.

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

As nouns, merit pay is a hyponym of earnings; that is, merit pay is a word with a more specific, narrower meaning than earnings:

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

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