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A supernova impostor is a bright transient with some appearance of a supernova (SN), but does not destroy the star. It can be thought of as an extreme nova of some kind. It is not as bright as a supernova, and some of the similarity is in the spectral energy distribution. There can be varying opinions about particular transients regarding whether it is a SN versus an impostor.
I suspect the term supernova impostor is now often used more-specifically for those with a particular mechanism, and other terms are being coined for various specific mechanisms. The general study of optical transients is developing quickly, which will continue as new technology is deployed, such as the upcoming roll-out of the Rubin Observatory.