What is the name of the magical rope that bound Fenrir?

The magical rope that bound Fenrir is called Gleipnir.

Gleipnir was created by dwarves after Fenrir broke two enormous bindings, Leyding and Dromi. Although it looked as thin and soft as a silk ribbon, it was stronger than any ordinary chain. The gods persuaded Fenrir to test it on the island of Lyngvi, but the wolf demanded a pledge of good faith. Týr placed his hand in Fenrir’s mouth; when the wolf realized he could not escape, he bit it off.

According to the Prose Edda, Gleipnir was made from six impossible ingredients: a cat’s footsteps, a woman’s beard, mountain roots, bear sinews, fish breath, and bird spittle. The myth explains why those things can no longer be found in the world.

Gleipnir is not the same as Gungnir, Odin’s spear, Draupnir, Odin’s multiplying ring, or Lævateinn, a mysterious weapon mentioned in later Norse tradition. Fenrir remains bound until Ragnarök, when he breaks free and kills Odin.