A positron was a subatomic particle, and is an antiparticle of the electron. Positrons belonged to a type of particle known as antileptons, the basis of antimatter.
The flow rate of positrons was the fourth and final step of a subspace infrared algorithm. (VOY: "Good Shepherd")
During an encounter in 2154 with the Orions, the Enterprise NX-01 found itself on the receiving end of a towing cable that was deployed from Harrad-Sar's barge. The Enterprise was able to break free after it sent a positron burst through its navigational deflector and along the towing cable, which resulted in the triggering of an energy cascade that successfully disabled the Orion's power systems. (ENT: "Bound")
In 2293, USS Excelsior fired a modulated positron beam at the Azure Nebula to ignite the sirillium gasses inside to disable Kang's battle cruiser. (VOY: "Flashback")
The Delta Rana warship used jacketed beams of antiprotons and positrons as its main weapon and fired beams with a firepower of forty megawatts and later four hundred gigawatts at the USS Enterprise-D in 2366. (TNG: "The Survivors")
Positrons could be detected using a positron emission sensor. Doctor Beverly Crusher used this device in 2367 while undertaking an autopsy of the crew of the USS Brattain. The sensor was used for brain tissue cross-sections. (TNG: "Night Terrors")
While exploring a class T cluster in 2376, an away team on the Delta Flyer found displaced positrons on a piece of hull that had been sheared off the ship. One possible cause was an impact from dark matter, though the results were inconclusive. (VOY: "Good Shepherd")