Psionic Matrix

From The StarCraft 2 Wiki

Protoss buildings and units, to a lesser extent, draw their energy from a great Psionic Energy Matrix that emanates from Aiur, the 'light' counterpart to the Dark Templar's Void.

While the Nexus provides a link to this matrix, Pylons are needed to actually tap into the energy required to provide Psionic energy (Psi) to new colonies. Each Pylon generates a short-ranged aura of Psi, which can provide the power needed by buildings and warp gates (initiated by Probe's Warp-beacons). 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 Psi to provide them with power.

[edit] Gameplay

Available Psi energy is displayed in the upper right-hand corner of the Main Screen, and selecting a Pylon or attempting to warp in a new building will display a blue aura with a hexagonal grid that represents the extent of the current Psionic Matrix. The StarCraft I auras use a singular light blue colour instead.

Also in StarCraft I, if a Protoss building has a supporting Pylon destroyed mid-warp, it will continue to warp in until finished. Afterwards it will become "unpowered".

It is unknown if this has changed with StarCraft II however it had been confirmed early during the development that units in mid-warp to a destroyed Pylon or Phase Prism will likely be lost with the cost credited.