The Collaborative International Dictionary of English v.0.48:

Gregorian \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.
   [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]

The Collaborative International Dictionary of English v.0.48:

Calendar \Cal"en*dar\, n. [OE. kalender, calender, fr. L.
   kalendarium an interest or account book (cf. F. calendrier,
   OF. calendier) fr. L. calendue, kalendae, calends. See
   Calends.]
   1. An orderly arrangement of the division of time, adapted to
      the purposes of civil life, as years, months, weeks, and
      days; also, a register of the year with its divisions; an
      almanac.
      [1913 Webster]

   2. (Eccl.) A tabular statement of the dates of feasts,
      offices, saints' days, etc., esp. of those which are
      liable to change yearly according to the varying date of
      Easter.
      [1913 Webster]

   3. An orderly list or enumeration of persons, things, or
      events; a schedule; as, a calendar of state papers; a
      calendar of bills presented in a legislative assembly; a
      calendar of causes arranged for trial in court; a calendar
      of a college or an academy.
      [1913 Webster]

   Note: Shepherds of people had need know the calendars of
         tempests of state. --Bacon.
         [1913 Webster]

   Calendar clock, one that shows the days of the week and
      month.

   Calendar month. See under Month.

   French Republican calendar. See under Vend['e]miaire.

   Gregorian calendar, Julian calendar, Perpetual
   calendar. See under Gregorian, Julian, and Perpetual.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Gregorian calendar
    n 1: the solar calendar now in general use, introduced by
         Gregory XIII in 1582 to correct an error in the Julian
         calendar by suppressing 10 days, making Oct 5 be called Oct
         15, and providing that only centenary years divisible by
         400 should be leap years; it was adopted by Great Britain
         and the American colonies in 1752 [syn: Gregorian
         calendar, New Style calendar]

The Free On-line Dictionary of Computing (30 December 2018):

Gregorian calendar

    The system of dates used by most of the world.  The
   Gregorian calendar was proposed by the Calabrian doctor
   Aloysius Lilius and was decreed by, and named after, Pope
   Gregory XIII on 1582-02-24.  It corrected the Julian calendar
   whose years were slightly longer than the solar year.  It also
   replaced the lunar calendar which was also out of time with
   the seasons.  The correction was achieved by skipping several
   days as a one-off resynchronisation and then dropping three
   leap days every 400 hundred years.  In the revised system,
   leap years are all years divisible by 4 but excluding those
   divisible by 100 but including those divisible by 400.  This
   gives a mean calendar year of 365.2425 days = 52.1775 weeks =
   8,765.82 hours = 525,949.2 minutes = 31,556,952 seconds.
   leap seconds are occasionally added to this to correct for
   irregularities in the Earth's rotation.

   (2007-01-10)