pillow slip vs pillowcase

pillow slip and pillowcase both are nouns.

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

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

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

Other synonyms of pillow slip include case, slip.

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