Proton

A proton was a subatomic particle. (TV: Doctor Who and the Silurians) It was one of the components of an atom, along with electrons and neutrons. (PROSE: The Infinity Doctors)

According to Jack Harkness, the Higgs particle was the "fundamental particle of existence", of which protons and other subatomic matter were made. The Large Hadron Collider accidentally opened a gateway to the neutron eaters reality, which was only able to be closed by protons and antiprotons being fired at each other. As a side-effect, Higgs particles were created. (AUDIO: Lost Souls)

Protons in technology
Several pieces of technology had "proton" in their name, most of which were used as weapons of one sort or another.

Weapons

 * Proton-beam projector (PROSE: The Great Mercury Plot)
 * Proton blaster (COMIC: Station Zero)
 * Proton cannon (COMIC: Second Wave, Extermination of the Daleks)
 * Proton flamethrower (AUDIO: The Torchwood Archive)
 * Proton missile (TV: Warriors of the Deep)

Other

 * Proton accelerator (TV: Doctor Who and the Silurians)
 * Reverse pulse proton engine (TV: Praxeus)

A "proton pack" was also a fictional piece of technology from the film Ghostbusters. (TV: The Eternity Trap)