plum vs damson plum

plum and damson plum both are nouns.

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

As nouns, damson plum is a hyponym of plum; that is, damson 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
damson plum: dark purple plum of the damson tree

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

plum (noun)	damson plum (noun)
a highly desirable position or assignment	dark purple plum of the damson tree
any of numerous varieties of small to medium-sized round or oval fruit having a smooth skin and a single pit	plum tree long cultivated for its edible fruit
any of several trees producing edible oval fruit having a smooth skin and a single hard stone	tropical American timber tree with dark hard heavy wood and small plumlike purple fruit

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