The process of
instantaneously relocating to another
location. Teleportation, the ability to
disappear and reappear immediately anywhere I choose is probably #1 on my
personal list of three wishes.
In AD&D, various types of teleportation are possibly available to players through magical items, artifacts, DM aribitration, and the spells teleport, teleport without error, dimension door, blink, and several others.
In NetHack, teleportation is an ability gained by wearing a ring of teleportation. However, teleportation is random unless you also wear a ring of teleport control or have intrinsic teleport control. Intrinsic teleportation (gained by eating nymph, leprechaun, or (sometimes) tengu corpses, and also by eating the corpse of the Wizard of Yendor) is also known by the name "teleportitis".