\Gre*go"ri*an\, a. [NL. Gregorianus, fr. Gregorius
Gregory, Gr. ?: cf. F. gr['e]gorien.]
Pertaining to, or originated by, some person named Gregory,
especially one of the popes of that name.
{Gregorian calendar}, the calendar as reformed by Pope
   Gregory XIII. in 1582, including the method of adjusting
   the leap years so as to harmonize the civil year with the
   solar, and also the regulation of the time of Easter and
   the movable feasts by means of epochs. See {Gregorian
   year} (below).
{Gregorian chant} (Mus.), plain song, or canto fermo, a kind
   of unisonous music, according to the eight celebrated
   church modes, as arranged and prescribed by Pope Gregory
   I. (called ``the Great'') in the 6th century.
{Gregorian modes}, the musical scales ordained by Pope
   Gregory the Great, and named after the ancient Greek
   scales, as Dorian, Lydian, etc.
{Gregorian telescope} (Opt.), a form of reflecting telescope,
   named from Prof. James Gregory, of Edinburgh, who
   perfected it in 1663. A small concave mirror in the axis
   of this telescope, having its focus coincident with that
   of the large reflector, transmits the light received from
   the latter back through a hole in its center to the
   eyepiece placed behind it.
{Gregorian year}, the year as now reckoned according to the
   Gregorian calendar. Thus, every year, of the current
   reckoning, which is divisible by 4, except those divisible
   by 100 and not by 400, has 366 days; all other years have
   365 days. See {Bissextile}, and Note under {Style}, n., 7.