Prunus domestica vs plum tree

Prunus domestica and plum tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
Prunus domestica	Yes	No	No	No
plum tree	Yes	No	No	No

As nouns, plum tree is a hypernym of Prunus domestica; that is, plum tree is a word with a broader meaning than Prunus domestica:

Prunus domestica: any of various widely distributed plums grown in the cooler temperate areas
plum tree: any of several trees producing edible oval fruit having a smooth skin and a single hard stone

Other hypernyms of Prunus domestica include plum.

Prunus domestica (noun)	plum tree (noun)
any of various widely distributed plums grown in the cooler temperate areas	any of several trees producing edible oval fruit having a smooth skin and a single hard stone