The phenomenon that a Laser exhibits sudden jumps of optical frequency, which are associated with transitions between different Modes of its resonator.
Mode hopping is a phenomenon which is mostly discussed in the context of single-frequency lasers. Such a laser may operate on a single ResonatorMode for some time, but then suddenly Switch to some other mode. This means that this other mode suddenly takes over all the optical power; for a short while, there may be power in both modes.
In Multimode operation of a laser, there may also be transitions between different sets of modes. However, simultaneous oscillation on many modes is then most common, and instead of complete mode hops there are often more continuous transitions, with the Optical Power being gradually redistributed. The dynamics can be further influenced by nonlinear effects such as spatial hole burning.
Mode hops can also involve higher-order modes, or modes with different Polarization in lasers with polarization-independent gain.