mainstay vs linchpin

mainstay and linchpin both are nouns.

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

As nouns, mainstay and linchpin are synonyms defined as:

mainstay and linchpin: a central cohesive source of support and stability

Other synonyms of mainstay include anchor, backbone, keystone, lynchpin.

mainstay (noun)	linchpin (noun)
the forestay that braces the mainmast	pin inserted through an axletree to hold a wheel on
a central cohesive source of support and stability	a central cohesive source of support and stability
a prominent supporter