"middle; being the middle part or midst; being between, intermediate," Old English mid, midd from Proto-Germanic *medja- (source also of Old Norse miðr, Old Saxon middi, Old Frisian midde, Middle Dutch mydde, Old High German mitti, German mitte, Gothic midjis "mid, middle"), from PIE root *medhyo- "middle."
By late Middle English probably felt as a prefix only, and now surviving in English only as a prefix (mid-air, midstream, etc.). Prefixed to months, seasons, etc. from late Old English. As a preposition, "in the middle of, amid" (c. 1400) it is from in midde or a shortened form of amid (compare midshipman) and sometimes is written 'mid.
"of or pertaining to the sea off the west coast of Africa," early 15c., Atlantyke, from Latin Atlanticus, from Greek Atlantikos "of Atlas," adjectival form of Atlas (genitive Atlantos), in reference to Mount Atlas in Mauritania (see Atlas). Applied since c. 1600 to the ocean between Europe and Africa, on one side, and the Americas on the other. As a noun late 14c., Athlant, from Old French Atlante.
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Definitions of mid-Atlantic
of a region of the United States generally including Delaware; Maryland; Virginia; and usually New York; Pennsylvania; New Jersey;