Teleportation is the transfer of matter from one point to another, more or less instantaneously.
Nightcrawler used it mainly as a defensive mechanism to avoid damage. He teleported to distract a robot in the Danger Room while Wolverine destroyed it. His teleporting can be quite unexpected as Kitty Pryde found out. He also used it to avoid combat with the Brotherhood of Mutant Terrorists in Asteroid M while he went after the Mutant Power Circuit. He then had to make one long teleportation so that the Scorpio Comet would destroy the Asteroid and avoid Earth. While he was entering Earth's atmosphere he teleported once again onto the Blackbird to avoid burning up.