  The term "ultraviolet (UV) burst" is introduced to describe small, intense,
transient brightenings in ultraviolet images of solar active regions. We
inventorize their properties and provide a definition based on image sequences
in transition-region lines. Coronal signatures are rare, and most bursts are
associated with small-scale, canceling opposite-polarity fields in the
photosphere that occur in emerging flux regions, moving magnetic features in
sunspot moats, and sunspot light bridges. We also compare UV bursts with
similar transition-region phenomena found previously in solar ultraviolet
spectrometry and with similar phenomena at optical wavelengths, in particular
Ellerman bombs. Akin to the latter, UV bursts are probably small-scale magnetic
reconnection events occurring in the low atmosphere, at photospheric and/or
chromospheric heights. Their intense emission in lines with optically thin
formation gives unique diagnostic opportunities for studying the physics of
magnetic reconnection in the low solar atmosphere. This paper is a review
report from an International Space Science Institute team that met in
2016-2017.
