earnings vs half-pay

earnings and half-pay both are nouns.

Word	Noun	Adjective	Verb	Adverb
earnings	Yes	No	No	No
half-pay	Yes	No	No	No

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

earnings (noun)	half-pay (noun)
something that remunerates	reduced wage paid to someone who is not working full time
the excess of revenues over outlays in a given period of time (including depreciation and other non-cash expenses)