pillow vs bolster

pillow and bolster both are nouns.

Word	Noun	Adjective	Verb	Adverb
pillow	Yes	No	Yes	No
bolster	Yes	No	Yes	No

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

pillow: a cushion to support the head of a sleeping person
bolster: a pillow that is often put across a bed underneath the regular pillows

Other hyponyms of pillow include bed pillow, long pillow.

pillow (noun)	bolster (noun)
a cushion to support the head of a sleeping person	a pillow that is often put across a bed underneath the regular pillows

pillow (verb)	bolster (verb)
rest on or as if on a pillow	add padding to
	support and strengthen
	prop up with a pillow or bolster