pillowcase vs pillow slip

pillowcase and pillow slip both are nouns.

Word	Noun	Adjective	Verb	Adverb
pillowcase	Yes	No	No	No
pillow slip	Yes	No	No	No

As nouns, pillowcase and pillow slip are synonyms defined as:

pillowcase and pillow slip: bed linen consisting of a cover for a pillow

Other synonyms of pillowcase include case, slip.

pillowcase (noun)	pillow slip (noun)
bed linen consisting of a cover for a pillow	bed linen consisting of a cover for a pillow