A muon is a short-lived subatomic particle classified as a lepton. Muons have similar properties to an electron, however they are some two hundred times heavier. Like electrons, they have antimatter counterparts as well.
If a muon feedback beam was initiated while a full intensity power transfer was being conducted, a buildup of muons would develop in the warp core. If that ship was to then go to warp speed, the warp engines would explode. It would require a level 3 diagnostic of the dilithium chamber to detect the muon particle buildup. (TNG: "The Next Phase")