Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Source : WordNet®

damson
     n : sweet dark purple plum [syn: {damson plum}]