mainstay vs pillar

mainstay and pillar both are nouns.

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

As nouns, mainstay and pillar are synonyms defined as:

mainstay (noun)	pillar (noun)
the forestay that braces the mainmast	(architecture) a tall vertical cylindrical structure standing upright and used to support a structure
a central cohesive source of support and stability	a vertical cylindrical structure standing alone and not supporting anything (such as a monument)
a prominent supporter	a fundamental principle or practice
	a prominent supporter
	anything that approximates the shape of a column or tower