beach plum

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A seacoast shrub (Prunus maritima) in the rose family, native to northeast North America and having white flowers and edible, plumlike fruits used in jams, jellies, and pies.

Wiktionary

1. n. A tree, Prunus maritima, bearing small plums, found along the US Atlantic coast from Maine to Maryland.

WordNet 3.0

1. n. small dark purple fruit used especially in jams and pies
2. n. seacoast shrub of northeastern North America having showy white blossoms and edible purple fruit