Portal (gateway)

A portal is an interdimensional vortex of energy that is used to connect dimensions.

Transportation use
A portal may be used for transport between two dimensions, but further research by the Resistance scientists at Black Mesa East (and workers at the Aperture Science Enrichment Center) revealed that portals could be used for transport within one dimension. This was a major discovery, as not even the Combine understood that portals could be used for instantaneous transport within one dimension and were forced to use much slower conventional means of transportation if they wanted to stay within a single dimension. It must be noted however, that human-made portals are able to accomplish this feat by using Xen as a relay; in the words of Judith Mossman, "we figured out how to use Xen as an unexpressed axis, effectively a dimensional slingshot so we can swing around the borderworld and come back to local space without having to pass through"; another thing to consider is that Barney Calhoun, during his brief visit to Xen, activated and calibrated a "focal point" which then used by Doctor Rosenberg to teleport outside of Black Mesa; modern teleporters (such as the Lambda Portal Reactor, the portals used by the Resistance or the Combine portal at Nova Prospekt) do not make use of this device, instead using a modulator to emulate it's effects. This method of using an "Unexpressed Axis" centered on Xen remove the need to travel through it or to use a focuser device (like the one activated by Barney Calhoun). Another notable fact is that through the use of the Aperture Science Handheld Portal Device, Chell could make objects exist in two dimensions simultaneously. The Black Mesa version of the Device, the Displacer, can also be used for dimensional travels, although how a handheld device is capable of teleporting itself and the user into Xen and back using internal power is a mystery.

Consequences
After the resonance cascade at Black Mesa, randomized portals opened often on Earth, sending in more Xen life forms. This randomized opening of portals obviously causes chaos, and is known as a Portal Storm.

Research
Portals were extensively studied by Earth's most intelligent human beings, as well as the Combine (though Earth had a better understanding of portals). The installations where portals were studied included Black Mesa, Black Mesa East, Aperture Laboratories, and White Forest.

Superportals
Evidently a far more powerful type of portal, Superportals are used to transport entire armies through dimensions.