Stone fruit, genus Prunus.
Plums (also known as gages) are stone fruits, genus Prunus, sub-genus Prunus. They can be distinguished from other Prunus (.eg. cherries and peaches) by a terminal bud and solitary side buds. The flowers are in groups of 1-5 on short stems. The stone is smooth and the fruit has a single groove down one side. Mature fruit also have a dusty coating which is easily rubbed off.
The sub-genus contains a number of species. These are divided into Old World Plums; New World Plums, and apricots. Apricot fruit have a velvet coating, and some authors consider them a separate sub-genus.
Use this tag for all questions about plums and gages. Use apricots for questions about apricots.