a. [ NL. Gregorianus, fr. Gregorius Gregory, Gr. &unr_;: cf. F. grégorien. ] Pertaining to, or originated by, some person named Gregory, especially one of the popes of that name. [ 1913 Webster ]

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. [ 1913 Webster ]

prop. n. 1. Pope Gregory XIII., born 1572, died 1585, the pope who introduced the modern calendar.
Syn. -- Gregory XIII, Ugo Buoncompagni. [ WordNet 1.5 ]

2. Pope Gregory I., born circa 540, died 604; the pope for whom Gregorian chants were named.
Syn. -- Gregory I, Saint Gregory I, Gregory the Great. [ WordNet 1.5 ]