A Repulsor is a particle weapon designed and built by Tony Stark. It was originally designed simply as a flight stabilizer for the upgraded flight capabilities of the Mark II. With the introduction of the Mark III, the Repulsor demonstrated that it could be used as a weapon, if the energy was focused into an energy beam. Becoming the primary weapons used in the Iron Man armor, they have been used in every version. They can be fired in small, concentrated, single bursts, or sustained in a high energy ray. The sixth armor could even reduce the repulsors into welding beams, allowed him to install an Arc Reactor for Stark Tower.
Repulsors were first introduced in Iron Man's Classic armor (the name given in the official video game) and has been used in every armor created since. Repulsors have also been integrated into the Insight Helicarriers, only used to achieve and sustain flight, and Stark's Iron Legion drones that appear in Avengers: Age of Ultron.