plum vs damson

plum and damson both are nouns.

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

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

plum (noun)	damson (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
any of several trees producing edible oval fruit having a smooth skin and a single hard stone

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