A sungrazing comet is a comet
(Sun-orbiting object with a coma
and possibly a tail)
whose orbit passes very close to the Sun.
There is no official definition, but one criterion is a perihelion
of no more than 0.01 astronomical unit (roughly within a solar radius from
the Sun's photosphere).
Often they evaporate during their closest approach,
the smaller they are, the more likely that is to happen.
On the order of 1500 have been observed.
Approaching close to the Sun makes them active,
and can make the large ones very prominent in the sky.
The majority of observed sungrazing comets are
in a comet family called the Kreutz comets.