wild apple vs crab apple

wild apple and crab apple both are nouns.

Word	Noun	Adjective	Verb	Adverb
wild apple	Yes	No	No	No
crab apple	Yes	No	No	No

As nouns, wild apple and crab apple are synonyms defined as:

wild apple and crab apple: any of numerous wild apple trees usually with small acidic fruit

Other synonyms of wild apple include crabapple.

wild apple (noun)	crab apple (noun)
any of numerous wild apple trees usually with small acidic fruit	small sour apple; suitable for preserving
	any of numerous wild apple trees usually with small acidic fruit
	any of numerous varieties of crab apples cultivated for their small acidic (usually bright red) fruit used for preserves or as ornamentals for their blossoms