Malus sylvestris vs crab apple

Malus sylvestris and crab apple both are nouns.

Word	Noun	Adjective	Verb	Adverb
Malus sylvestris	Yes	No	No	No
crab apple	Yes	No	No	No

As nouns, crab apple is a hypernym of Malus sylvestris; that is, crab apple is a word with a broader meaning than Malus sylvestris:

Malus sylvestris: wild crab apple native to Europe; a chief ancestor of cultivated apples
crab apple: any of numerous wild apple trees usually with small acidic fruit

Other hypernyms of Malus sylvestris include crabapple, wild apple.

Malus sylvestris (noun)	crab apple (noun)
wild crab apple native to Europe; a chief ancestor of cultivated apples	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