Magic Lantern is a firmware add-on for various Canon digital single-lens reflex (DSLR) cameras and the EOS M. It adds features for DSLR filmmaking and still photography, and is free and open-source. There has not been an update of any of the various models' firmware since July 2018, and no models since then have been written.
Magic Lantern was originally written for the Canon EOS 5D Mark II by Trammell Hudson in 2009 after he reverse-engineered its firmware. He ported it to the Canon EOS 550D in July 2010. There are now versions for many other Canon DSLRs and the current principal developer is known as A1ex.
Because installing Magic Lantern does not replace the stock Canon firmware or modify the ROM but rather runs alongside it, it is both easy to remove and carries little risk. The camera checks a "boot flag" in its re-writable memory, and if set, reads from a memory card to get the additional firmware routines. Each time the camera is started, there is an option to disable Magic Lantern.