region or island at northernmost part of the world, Old English, from Latin, from Greek Thyle "land six days' sail north of Britain" (Strabo, quoting a lost portion of a work by Polybius, itself based on a lost account of a voyage to the north by 4c. B.C.E. geographer Pytheas). The identity of the place and the source of the name have sparked much speculation; Polybius doubted the whole thing, and since Roman times the name has been used in a transferred sense of "extreme limits of travel" (Ultima Thule).
The barbarians showed us where the sun set. For it happened in those places that the night was extremely short, lasting only two or three hours; and the sun sunk under the horizon, after a short interval reappeared at his rising. [Pytheas]
The name was given to a trading post in Greenland in 1910, site of a U.S. air base in World War II.