The term ultima Thule in medieval geographies denotes any distant place located beyond the "borders of the known world". Sometimes it is used as a proper noun (Ultima Thule) as the Latin name for Greenland when Thule is used for Iceland.
The Greek explorer Pytheas is the first to have written of Thule, doing so in his now lost work, On the Ocean, after his travels between 330 BC and 320 BC. He supposedly was sent out by the Greek city of Massalia to see where their trade-goods were coming from.
During the Middle Ages the name was used first of all to denote Iceland, such as by Dicuil, by the British monk Venerable Bede in De ratione temporum, by the Landnámabók, by the anonymous Historia Norwegie and by the German bishop Bremen's Deeds of Bishops of the Hamburg Church, where they cite ancient writers' usage of Thule but also new knowledge since the end of antiquity. All these authors also understood that other islands were situated to the north of Britain.