Phase weapon
From Memory Alpha, the free Star Trek reference
Phase weapons, or phase-modulated energy weapons, were a type of directed energy device, or particle weapon, used by the Earth Starfleet. They were first used in the early 2150s, and had both non-lethal stun and lethal settings.
Malcolm Reed showed Jonathan Archer a phase pistol for the first time in 2151 shortly before the first time Archer faced down Silik, recovering the Klingon agent Klaang from Silik's Suliban Helix. (ENT: "Broken Bow")
The Enterprise NX-01 was refitted with internally-mounted retractable phase cannons during the first year of its interstellar exploration mission. (ENT: "Silent Enemy")
Spock remarked that the Earth-Romulan War was fought with atomic weapons. (TOS: "Balance of Terror") Given that phase weapons are fundamentally different technology, it can be assumed Earth Starfleet was only able to outfit Enterprise, Columbia, and a few other ships with such weapons, falling back to nuclear-based spatial torpedoes for the rest of the fleet.
