crab apple vs apple tree

crab apple and apple tree both are nouns.

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

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

crab apple: any of numerous wild apple trees usually with small acidic fruit
apple tree: any tree of the genus Malus especially those bearing firm rounded edible fruits

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

crab apple: 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
apple tree: any tree of the genus Malus especially those bearing firm rounded edible fruits

crab apple (noun)	apple tree (noun)
small sour apple; suitable for preserving	any tree of the genus Malus especially those bearing firm rounded edible fruits
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