An Intermediate Luminosity Optical Transient (ILOT) is an astronomical object which undergoes an optically detectable explosive event with an absolute magnitude (M) brighter than a classical nova (M ~ −8) but fainter than that of a supernova (M ~ −17). That nine magnitude range corresponds to a factor of nearly 4000 in luminosity, so the ILOT class may include a wide variety of objects. The term ILOT first appeared in a 2009 paper discussing the nova-like event NGC 300 OT2008-1.^[1] As the term has gained more widespread use,^[2] it has begun to be applied to some objects like KjPn 8 and CK Vulpeculae for which no transient event has been observed, but which may have been dramatically affected by an ILOT event in the past.^[3][4] The number of ILOTs known is expected to increase substantially when the Vera C. Rubin Observatory becomes operational.

A very wide variety of objects have been classified as ILOTs in the astronomical literature. Kashi and Soker proposed a model for the outburt of ASASSN-15qi,^[5] in which a Jupiter-mass planet is tidally destroyed and accreted onto a young main sequence star.^[6] Red novae, believed be caused by the merger of two stars, are classified as ILOTs.^[7] Some luminous blue variables, such as η Car have been classified as ILOTs.^[8] Some objects which have been classified as failed supernovae may be ILOTs.^[9] The common thread tying all of these objects together is a transfer of a large amount of mass (0.001 M_⊙ to a few M_⊙) from a planet or star to a companion star, over a short period of time, leading to a massive eruption. That large range in accretion mass explains the large range in ILOT event brightness.^[10]