arctic (adj.)

late 14c., antartyk "opposite to the north pole" (adj.), from Old French antartique, from Medieval Latin antarcticus, from Greek antarktikos "opposite the north," from anti- "opposite" (see anti-) + arktikos "arctic" (see arctic).

The first -c- ceased to be pronounced in Medieval Latin and was dropped in Old French. Modern English restores it in spelling from 17c. Also from late 14c. as a noun (with capital A-), "region around the South pole of the sky or the southern regions of the Earth."

1550s in astronomy, in reference to a celestial circle, a line around the sky which, in any location, bounds the stars which are ever-visible from that latitude (in the Northern Hemisphere its center point is the celestial north pole); the concept goes back to the ancient Greeks, for whom this set of constellations included most prominently the two bears (arktoi), hence the name for the circle (see arctic). In Middle English it was the north cercle (late 14c.).

In geography, from 1620s as "the circle roughly 66 degrees 32 minutes north of the equator" (based on obliquity of the ecliptic of 23 degrees 28 minutes), marking the southern extremity of the polar day, when the sun at least theoretically passes the north point without setting on at least one summer day and does not rise on at least one winter one.