A stellar flare is a sudden brightening of a star. In the case of the Sun, it is called a solar flare.
Stellar flares vary in brightness and the energy presumed to be released. They are generally assumed to be the results of magnetic reconnection.
Stars sometimes have periods of time when there are more and/or larger flares, and are said to be in a flaring state (or flare state) versus their other quiescent state. (These terms are also used for other astronomical bodies that emit or trigger emission of electromagnetic radiation.)
A superflare or megaflare is a flare with as much as a million times the energy of a typical solar flare, having been observed with energies of 2 × 1033 to 2 × 1038 ergs. One theory is that a star's magnetic field interacts with that of a giant planet that has more of a magnetic field than Jupiter. Kepler has observed some.
Some stars that have extra-solar planets within their habitable zones (as far as equilibrium temperature is concerned) may still be unlikely to support life because of too-frequent flares with considerable ultraviolet.