A nova is a white dwarf explosion less energetic than a supernova. It is a thermonuclear explosion (CNO cycle) of surrounding compressed hydrogen accreted from a binary companion. Novae appear as brighter stars that gradually lose their brightness, losing 2 magnitudes in 25 to 80 days (for faster to slower novae). Novae are regularly tracked in Andromeda as well as the Milky Way.
The term classical nova (CN) is sometimes used to emphasize a supernova is not what is meant. It also is sometimes used to distinguish a one-time nova from a recurrent nova, i.e., a star that undergoes repeated novae.
An X-ray nova is a transient ascribed to material falling into a black hole or neutron star, material from a companion star or perhaps a whole star.