mainstay vs booster

mainstay and booster both are nouns.

Word	Noun	Adjective	Verb	Adverb
mainstay	Yes	No	No	No
booster	Yes	No	No	No

As nouns, booster is a hypernym of mainstay; that is, booster is a word with a broader meaning than mainstay:

Other hypernyms of mainstay include admirer, champion, friend, protagonist, supporter.

mainstay (noun)	booster (noun)
the forestay that braces the mainmast	an additional dose that makes sure the first dose was effective
a central cohesive source of support and stability	the first stage of a multistage rocket
a prominent supporter	an amplifier for restoring the strength of a transmitted signal
	a thief who steals goods that are in a store
	someone who is an active supporter and advocate
	a person who backs a politician or a team etc.