An arc reactor can refer to two different things: a fictional power source in the Marvel Cinematic Universe or a theoretical design for a compact fusion reactor developed by the Massachusetts Institute of Technology (MIT) .

In the Marvel Cinematic Universe, the arc reactor is a device initially designed by Howard Stark and later adapted by his son, Tony Stark (Iron Man), to provide power for his suits and the Avengers Tower. It is essentially a miniature fusion reactor that harnesses energy by removing electrons from hydrogen atoms, creating an ion plasma that is the ultimate source of energy.

On the other hand, the ARC fusion reactor is a design for a compact fusion reactor developed by MITs Plasma Science and Fusion Center. It aims to achieve an engineering breakeven of three and uses rare-earth barium copper oxide (REBCO) high-temperature superconductor magnets in place of copper wiring or conventional low-temperature superconductors. The first machine planned to come from the project is a scaled-down demonstrator named SPARC (as Soon as Possible ARC) . The name and design were inspired by the fictional arc reactor built by Tony Stark in the comic books, as the students who worked on the project were MIT fans of the Marvel Cinematic Universe.