The z-particle is a subatomic particle that has a zero electrical charge and is its own antiparticle.
Z-particles were created when Armus used his powers. The crew of the USS Enterprise-D used sensor scans of these occurring as an indicator of Armus' power level when they encountered him at Vagra II in 2364. (TNG: "Devil's Due").
When Geordi La Forge tried to find out what caused the mysterious disappearance of his former crewmates of the USS Victory on Tarchannen III, he scanned for z-particles, among other things. No z-particles were detected, however. (TNG: "Identity Crisis")