A plum is a fruit of the subgenus Prunus of the genus Prunus. The subgenus is distinguished from other subgenera (peaches, cherries, bird cherries, etc.) in the shoots having terminal bud and solitary side buds (not clustered), the flowers in groups of one to five together on short stems, and the fruit having a groove running down one side and a smooth stone (or pit).

Mature plum fruit may have a dusty-white waxy coating that gives them a glaucous appearance. This is an epicuticular wax coating and is known as "wax bloom". Dried plum fruits are called "dried plums" or prunes, although, in many countries (Li hing mui for instance), prunes are a distinct type of dried plum having a wrinkled appearance.