That’s the term for light emitted by collapsing bubbles of ideal gas (ie, argon) in water.The light pulses are on the order of ~50 picoseconds. Lots of energy is produced, primarily driven by bremsstrahlung and also some errant chemistry from water molecules stuck inside the bubble. The good news is that physicists finally kind of understand it well enough to build simple models of it. The bad news is that it isn’t going to help us achieve cold fusion.