bolster vs pillow

bolster and pillow both are nouns.

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

As nouns, pillow is a hypernym of bolster; that is, pillow is a word with a broader meaning than bolster:

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

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

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