pillar vs pilaster

pillar and pilaster both are nouns.

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

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

pillar: (architecture) a tall vertical cylindrical structure standing upright and used to support a structure
pilaster: a rectangular column that usually projects about a third of its width from the wall to which it is attached

Other hyponyms of pillar include atlas, telamon, caryatid, newel, pile, piling, spile, stilt, support column.

pillar (noun)	pilaster (noun)
(architecture) a tall vertical cylindrical structure standing upright and used to support a structure	a rectangular column that usually projects about a third of its width from the wall to which it is attached
a vertical cylindrical structure standing alone and not supporting anything (such as a monument)
a fundamental principle or practice
a prominent supporter
anything that approximates the shape of a column or tower