The Collaborative International Dictionary
damask plum
Damson \Dam"son\ (d[a^]m"z'n), n. [OE. damasin the Damascus plum, fr. L. Damascenus. See Damascene.] A small oval plum of a blue color, the fruit of a variety of the Prunus domestica; -- called also damask plum.
Wiktionary
damask plum
n. (cx archaic English) damson (the fruit)