Elerium, also known as Element 115, is a strange element used in the First Alien War as a power source for alien technology. It is an incredibly rare substance that does not occur naturally on Earth, but has been found to exist in places elsewhere in the solar system, like Mars.
Elerium is used to fuel alien antimatter reactors on UFOs, and power plasma weapons and other equipment. When bombarded by subatomic particles, a reaction causes it to generate antimatter. Because of the energy released by matter/antimatter reactions and the efficient matter-to-energy conversion ratio, Elerium-powered reactors are many times as powerful than conventional nuclear reactors. This and the fact that all alien technologies relied on Elerium power sources made it prized substance to X-Com, and collecting as much Elerium as possible became a high priority.
After the Alien War, because mankind had lost its only means of obtainin Elerium, use of alien technology was discontinued. Some suggested salvaging UFO wrecks on the seabed, but unfortunately Elerium becomes inert when exposed to seawater. Because of this, Elerium-based technologies were not used by X-Com during the Second Alien War.
Following the global disaster caused by the destruction of T'leth, Elerium mines established on Mars and other places in the solar system allowed mankind to use the old alien technologies once again. Radical advances in technology following this allowed them to construct metropolitan cities that would allow humanity to survive on post-apocalyptic Earth. Elerium became even more precious than gold in this period, because of its rarity and importance to humanity.