Alpha Meson

Alpha Meson was a type of energy.

It was used as "alpha meson phosphor", the presence of which could provide a light source throughout a building without the need for bulbs or openings to natural light. (TV: The Tomb of the Cybermen)

River Song carried an Alpha Meson pistol, capable of disintegrating a wooden cabinet (TV: The Pandorica Opens) and destroying a weakened Dalek. (TV: The Big Bang)

Behind the scenes
In The Tomb of the Cybermen, the energy was pronounced as "May-zon", while in The Big Bang, it was pronounced as "Mee-zon".