Damson n. : A small oval plum of a blue color, the fruit of a variety of the Prunus domestica; -- called also damask plum.

Example:
Damson and elderberry rum what do you think will it taste nice?

Taxonomy
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Defination from the web:
damson derives from the Latin prunum damascenum, "plum of Damascus" ... The damson was introduced into the American colonies by English settlers before the American Revolution and are regarded as thriving better in ...