booster vs mainstay

booster and mainstay both are nouns.

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

As nouns, mainstay is a hyponym of booster; that is, mainstay is a word with a more specific, narrower meaning than booster:

booster (noun)	mainstay (noun)
an additional dose that makes sure the first dose was effective	the forestay that braces the mainmast
the first stage of a multistage rocket	a central cohesive source of support and stability
an amplifier for restoring the strength of a transmitted signal	a prominent supporter
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.