INTRODUCTION Superluminous supernovae (SLSNe), as the name attests, are supernovae that are brighter than usual. As a result of the overly broad name, the category is a catch-all describing several classes of supernovae – some with hydrogen, some without, some interacting, some probably not. A few authors have defined SLSNe as those brighter than M = −21 at peak, though this arbitrary cut could leave out related physical phenomena. Instead, I define SLSNe as luminous SNe which cannot be explained by the power sources fueling traditional (Types I and II) supernovae: radioactive decay from a moderate amount of elements synthesized in the explosion, the energy deposited by a shock unbinding the star, or interaction with moderate but obvious amounts of circumstellar material (CSM) previously lost by the supernova progenitor or a companion. This last point creates a gray area. Should Type IIn supernovae count as SLSNe? Type IIn supernovae are those with a strong blue continuum at early times, and narrow and intermediate width hydrogen emission lines at some points in their spectroscopic evolution. They are thought to be the collapse of massive stars whose ejecta shock CSM. On one hand, they have been recognized as a class since the 1980s, a large and diverse one, and the source of their luminosity is not a mystery. On the other, some SNe IIn are so bright that they have been considered SLSNe [e.g. SN 2006gy , which reached a peak absolute magnitude of −22]. A complicating factor is that interaction should be considered as a possible power source for SLSNe, whether or not the spectra show narrow lines. Here I compromise – I will generally not include clear SNe IIn, as their power source is not a mystery. However, I will mention a few extraordinary cases where appropriate, and discuss interaction as a possible power source.
