plum vs greengage plum

plum and greengage plum both are nouns.

Word	Noun	Adjective	Verb	Adverb
plum	Yes	No	No	Yes
greengage plum	Yes	No	No	No

As nouns, greengage plum is a hyponym of plum; that is, greengage plum is a word with a more specific, narrower meaning than plum:

plum: any of numerous varieties of small to medium-sized round or oval fruit having a smooth skin and a single pit
greengage plum: sweet green or greenish-yellow variety of plum

Other hyponyms of plum include damson, damson plum, greengage, beach plum, sloe, Victoria plum, cherry plum.

plum (noun)	greengage plum (noun)
a highly desirable position or assignment	sweet green or greenish-yellow variety of plum
any of numerous varieties of small to medium-sized round or oval fruit having a smooth skin and a single pit
any of several trees producing edible oval fruit having a smooth skin and a single hard stone

plum (adverb)	greengage plum (adverb)
exactly
completely; used as intensifiers