beach plum

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A shrub (Prunus maritima) in the rose family, found along the Atlantic coast from New Brunswick to Maryland and having white flowers and edible fruits used in jams and jellies.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun small dark purple fruit used especially in jams and pies
• noun seacoast shrub of northeastern North America having showy white blossoms and edible purple fruit