plum vs greengage

plum and greengage both are nouns.

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

As nouns, greengage is a hyponym of plum; that is, greengage 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: sweet green or greenish-yellow variety of plum

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

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