merit pay vs salary

merit pay and salary both are nouns.

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

As nouns, salary is a hypernym of merit pay; that is, salary 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)
salary: something that remunerates

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

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