Teleportation grid

The Teleportation Grid is a piece of Forerunner technology equipped to several important Installations.

Despite its name, the teleportation grid does not actually "teleport" its users; rather, it encases them in a compressed slipstream field, transporting them from one position to another almost instantaneously. This happens much in the same way that a spaceship uses slipspace to travel between star systems - time passes for the individual, but due to the short distances involved it is negligible.

Both Installation 04 and Installation 05 were equipped with teleportation grids, allowing their Monitors to reach vital areas rapidly, and supervise their Sentinel charges. Onyx also possessed a teleportation grid. Although it is uncertain, it's likely that even the Ark has one.

Installation 04
Installation 04's teleportation grid, was used extensively during the course of the Battle of Installation 04. The Installation's Monitor, 343 Guilty Spark, first encountered the Master Chief outside of a Flood Research Facility, and used the Teleportation Grid to transport them to the Installation's Library, in order to retrieve the Activation Index. After the Index was retrieved, the Monitor used the grid again to transport the two of them to the Control Room to activate the Halo's final weapon. During the Master Chief's efforts to neutralize the Installation's primary firing sequence, Cortana tapped into the teleportation grid, using his Mark V MJOLNIR armor’s fusion reactor to power a single jump, to a downed Covenant cruiser ; however, Cortana's relative inexperience with the grid resulted in reorientation difficulties.

Installation 05
Installation 05's teleportation grid was unavailable during the Battle of Installation 05 as it was being controlled by the Gravemind (formed beneath that Installation's Library facility) through the Installation's captured Monitor, 2401 Penitent Tangent. Managing to ensnare both the Master Chief and the Arbiter, it formed a tentative alliance with the two enemies in order to stop the rings activation - teleporting the Master Chief aboard High Charity, in orbit above the Installation, and the Arbiter near to the Control Room. The fact that it didn't, or couldn't, use the same grid to teleport its own Flood Combat Forms to those locations indicates that use of it is restricted to non-Flood life forms and constructs.

Onyx
The Shield World Onyx also possessed a teleportation grid, used by Doctor Halsey to transport the Spartan IIs and IIIs to a safe area, away from Covenant activity. Later, she used it to send Spartan teams to disable a Sentinel production factory before transporting them again to the center of the planet, where a portal leading to a Micro Dyson Sphere was found. They then used the Sphere as a shelter from the Covenant army invading the facility.

Trivia

 * In all Halo games, the teleportation field can be seen when in Co-Op. This occurs when one player has hit a loading zone, but the other has not. The player that has not hit the loading zone is teleported to where the first player is located.
 * This also happens in Halo 3 when you flip inside a vehicle. It can be viewed easily in theater mode.