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is an unincorporated area and census-designated place in Prince George's County, Maryland, United States. The population was 14,998 at the 2000 census. It includes the communities of Adelphi Hills, Buck Lodge, Chatham, Cool Spring Terrace, and White Oak Manor.
The unincorporated Adelphi community takes its name from the historic Adelphi Mill, established in 1796 along the Northwest Branch of the Anacostia River. The Adelphi Mill is administered by the Maryland-National Capital Park and Planning Commission and continues to attract visitors and can be rented for special functions. During the 19th century, George Washington Riggs acquired much of the area northeast of Washington, D.C. as his Green Hill estate in the Chillum Manor district. That estate included present day Adelphi. In the early 1920s, part of the area was acquired by Leander McCormick-Goodhart as part of his Langley Park estate. Labor organizer Mary Harris "Mother" Jones died in 1930, at the farm of Walter and Lillie May Burgess near Powder Mill and Riggs Roads in present day Adelphi. A marker was erected by the Maryland State Highway Administration commemorating Mother Jones at her death site.
Unlike its nearby neighbors Hyattsville and College Park, the area remained relatively undeveloped until immediately following World War II. At that time, subdivisions quickly developed. To distinguish it from the other neighboring unincorporated West Hyattsville communities of Lewisdale and Langley Park, the growing subdivisions banded together in the mid-1950s and adopted the name "Adelphi" to reflect their commitment to the preservation of the historic mill of the same name. These subdivisions now include Adelphi Hills, Adelphi Knolls, Buck Lodge, Chatham, Cool Spring Terrace, Heitmuller Estates, Hillandale Forest, Knollwood, Riggs Hill Manor, and White Oak Manor. A community focal point originally developed in the late 1950s is the Adelphi Pool.
In 1971, the Harry Diamond Laboratories was established on in the northern part of Adelphi. That facility continues as the Adelphi Laboratory Center, Army Research Laboratory (ARL); the U.S. Army's corporate research laboratory. Other adjacent federal facilities include the National Archives at College Park and headquarters of the Food and Drug Administration.