Definitions:

1. A symbol of the most remote, extreme, or mysterious location. NOUN
   • He sought his own Ultima Thule, a place where he could start over.
2. A term originally used in classical and medieval literature to describe the farthest possible place, often beyond the known world. NOUN
   • Ancient geographers believed Ultima Thule lay beyond the icy seas of the north.
3. Term used by ancient geographers and Roman poets to refer to lands beyond the edges of their maps. NOUN
   • To the Romans, Ultima Thule represented the edge of the known world.

Synonyms:

• ends of the earth
• final frontier
• outermost limit
• unknown land