Psionic blade

A psionic blade (or psi blade) is a type of protoss close-combat weapon. It is generated by channeling psionic energy, possibly by a combination of biological and mechanical means. Blades manifest themselves through the bracers on power suits and light armor suits and are a luminescent blue-white in color.

On occasion high templar have used these weapons.

The Dark Templar have further refined psi blades into much more powerful warp blades.

Terran Dominion scientists have developed similar devices for use by ghosts and similar soldiers.

Trivia
In StarCraft:_Queen_of_Blades, Tassadar is described to have been able to create psi blades without bracers.

In StarCraft II, it was originally planned that their psi-blades be colored in accordance with team color, but fan reaction shot the idea down, and a compromise was never reached.