A plum or gage is a stone fruit tree in the genus Prunus, subgenus Prunus. The subgenus is distinguished from other subgenera (peaches, cherries, bird cherries, etc) in the shoots having a terminal bud and the side buds solitary (not clustered), the flowers being grouped 1-5 together on short stems, and the fruit having a groove running down one side, and a smooth stone.
The damson or damson plum (Prunus domestica subsp. insititia, or sometimes Prunus insititia) is an edible drupaceous fruit, a subspecies of the plum tree. It is also sometimes known as the Damask Plum (but is slightly different from the Bullace).