Pylon

A pylon is a protoss structure needed for a probe to be able to warp in most protoss buildings (other pylons, nexuses and assimilators being the exceptions).

Overview
Protoss structures and units, to a lesser extent, draw their energy from a great psionic energy matrix that emanates from Aiur. While a nexus provides a link to this matrix, khaydarin crystal-based pylons are needed to actually tap into the energy required to provide psionic energy to new colonies. Each pylon generates a short-ranged aura of energy, which can provide the power needed by structures and warp gates.

Pylons project a strong magnetic field which makes the ground beneath them impenetrable by "mortal means".

The dark templar have their own version of the pylon in the form of the dark pylon. As circumstances grow increasingly dire for the protoss, the dark pylon has entered general use for the protoss alongside the regular pylon.

Pylon-Based Defenses
Large clusters of pylons were once used to generate an electromagnetic field that prevented zerg spire units from maneuvering. The clusters were based at Talematros, capital of Shakuras, before Infested Duran's forces destroyed them.

Gameplay
If a protoss building loses its connection to the psionic matrix, it will shut down until it is reconnected, and new units cannot be gated in if there is insufficient psionic energy to provide them with power. Unpowered buildings will display it when selected, and makes a particular sound instead of the building's own sound.

Its the only building in the game that do not leave debris when destroyed (without counting the vespene geyser buildings).

StarCraft II
In StarCraft II, the pylon is far less resilient than its predecessor. However, the protoss can also use warp prisms to support the structures if a pylon is lost.

Protoss warp gate units can be warped in within the psionic matrix created by pylons.

Putting a pylon in the central map area, known as "proxy pylon", in order to warp-in units nearer the enemy base is a commonly used strategy.

You Must Construct Additional Pylons
If a player attempts to create a unit when they don't have enough psi, they will hear an error message from the protoss advisor; "You must construct additional pylons".

Blizzard Entertainment employees have used the phrase to comical effect. The 404 error pages at www.starcraft2.com sometimes use the phrase.

The phrase "you must construct additional pylons" has become extremely popular on some areas of the internet, such as YTMND.