pillar vs mainstay

pillar and mainstay both are nouns.

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

As nouns, pillar and mainstay are synonyms defined as:

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