Teleportation

Teleportation is the transfer of matter from one place to another, more or less instantaneously, either by paranormal means or through technological artifice.

Use in the Half-Life series
As in many first-person shooter video games (such as the Doom or the Quake series), teleportation is one of the recurring themes of the Half-Life series.

Half-Life and its expansions
Teleportation was studied in the Black Mesa Research Facility, where functioning teleport where heavily used to travel to the border world of Xen, among others. Both Gordon Freeman (through the Black Mesa's Lambda Core teleport) and Barney Calhoun traveled to Xen to a teleport, after an experiment conducted by Freeman and other scientists fails and creates the Resonance Cascade, teleporting Xen creatures into Earth and starting the invasion that will finally lead to the Seven Hour War and the enslavement of humanity by the Combine.

Half-Life 2 and its episodes
At the start of the game, Isaac Kleiner and Eli Vance had been working on teleportation and were finally able to rebuild two teleports to connect Kleiner's Lab to Black Mesa East. At the start of the game, it is inaugurated and Alyx Vance travels safely to Black Mesa East. When Freeman's turn arrives, Lamarr, Kleiner's pet Headcrab, jumps into the teleport, which creates a malfunction, sending Gordon to several random places, such as the Wasteland, a lake where swims an Ichthyosaur, Black Mesa East and even Breen's office in the Citadel. Gordon finally gets to the exterior of Kleiner's Lab and is force to join Black Mesa East through another means.

When Gordon Freeman and Alyx Vance reach Eli Vance prisoner in the Nova Prospekt Depot, they force Judith Mossman to help them bring Eli to Kleiner's Lab through the Combine teleport installed in the lower sections of Nova Propekt. While they are not watching, Mossman changes the coordinates and leaves with Vance to the Citadel. Then Gordon and Alyx must reset the coordinates to reach Kleiner's Lab, while Combine forces are invading the teleport room. The teleportation is successful, but the teleporter went unstable during the process and exploded just after the teleportation was done, destroying the surroundings. Gordon and Alyx arrive at Kleiner's Lab a week later, while for them it just took seconds. In that timeframe, rebellion has started in City 17.