A form of eruptive variable that exhibits abrupt, unpredictable flares with a rise time of seconds and a decay time of minutes. Flare stars have a Ke- or Me-type spectrum and a strong magnetic field. Although the term is generally taken to be synonymous with UV Ceti stars, certain BY Draconis stars also exhibit flare activity. These flares are, in principle, the same kind of phenomenon as solar flares, but with much greater energies involved, perhaps by a factor of 1000.