mid-Atlantic (n.)

by 1804, "the middle of the Atlantic Ocean," from mid (adj.) + Atlantic. In U.S. history, Mid-Atlantic states in reference to the middle states on the Atlantic coast is by 1842.

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Definitions of mid-Atlantic from WordNet

mid-Atlantic (adj.)
of a region of the United States generally including Delaware; Maryland; Virginia; and usually New York; Pennsylvania; New Jersey;
Synonyms: middle Atlantic