"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.
early 15c., Atlantyke, "of or pertaining to the sea off the west coast of Africa," from Latin Atlanticus, from Greek Atlantikos "of Atlas," adjectival form of Atlas (genitive Atlantos) as used in reference to Mount Atlas in Mauritania (see Atlas). The name has been extended 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.
<a href="https://www.etymonline.com/word/mid-Atlantic">Etymology of mid-Atlantic by etymonline</a>
Harper, D. (n.d.). Etymology of mid-Atlantic. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/mid-Atlantic