In X-Com: UFO Defense (Enemy Unknown), the first installment of the iconic X-Com series, the most common weapon used by the alien soldiers were deadly energy weapons that fired beams of hyper-accelerated matter, or plasma. These plasma weapons could burn through reinforced steel or titanium and disintegrate anything softer than the aforementioned alloys, like wood, cloth, or flesh and bone.
These weapons use a highly-advanced Elerium-powered particle accelerator. The ammo clips they use are actually compact Elerium batteries that feed power to the accelerator.
There are several variations of Plasma weaponry. There are three handheld Plasma guns, a hovertank mounted with a Plasma cannon, the Plasma cannon that can be mounted on X-Com aircraft and the Plasma Defense base upgrade that defends the base from UFO attacks with an array of Plasma cannon turrets.
The most basic Plasma weapon, yet it's more easy to carry than its heavier cousins. A small pistol that fires Plasma beams that are very light but still lethal to anything that's not behind a wall of Alien Alloy.
An average-sized Plasma weapon, more powerful than the Pistol, and still easy to handle.
The Alien equivalent of the man-made Heavy Cannon, but its excellent versatility is botched in favor of sheer firepower. It's very heavy, true to the name, and can fire powerful Plasma beams that can reduce alien alloy to molten metal in several concentrated shots.
The first hovertank available to the X-Com arsenal. It is a heavy weapons platform that is capable of hovering above the ground by using a variation of a UFO's propusion system. This hovertank is mounted with a devastating Plasma beam that tears enemies to shreds.