d.c.
abbreviation of direct current, attested from 1898.
D.C.
abbreviation of District of Columbia, apparently not widely used before 1820, but eventually it became necessary to distinguish the place from the many other "Washingtons" in America. The city and the district were named in 1791 (at first known as Territory of Columbia; the territory was organized as a "district" in 1801), but the towns within it (Washington, Georgetown, Alexandria) remained separate municipalities and at one time all took D.C. The district was effectively organized as a unitary municipality in 1871.