Psi

From The StarCraft 2 Wiki

Psi is the physical measurement of Psionic Matrix in the game for Protoss players to show how many units they can support before warping in more Pylons and/or Nexus. The equivalent measurements for the other races are Terran supply and Zerg control.

If a player loses enough Pylons that the total amount of Psi becomes lower than the amount used by units, it will be impossible to warp in more units. This will not impair units that are already warped in, or affect units that have been brought to the game by the use of triggers.

Psi is a representation of the level of psionic support available to run the pseudo-psionic machinery of the Protoss.