A character who has the ability to travel between different alternate dimensions. Could also extend to characters with any kind of trans-world hopping ability. Said ability can be either inherent (the character has the power to do it) or external (they use some sort of device to do it). The important part is that it's specific to a character or characters. A dimension-travelling device that any character can make use of doesn't count.
The only example I can think of at the moment is Sliders
, which is one of the most well known examples. Quantum Leap
could also count.