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).
However, I've seen references implying more generous
criteria, with perihelion-limits seemingly
somewhere in the 0.015 to 0.06 AU range.
Often these comets 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 (Kreutz sungrazer comets)
that have a perihelion on the order of 0.005 AU,
and an orbital period of around 750 years,
and orbital inclination of 144 degrees, i.e., retrograde.
Other sunzgrazer comet families:
Meyer sungrazer comets (0.035 AU, a decade, 72° ).
Marsden sungrazer comets (0.044 AU, 5.6 years, 13° ).
Kracht sungrazer comets (0.049 AU, 3-4 years, 26° ).