Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.

   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.

   Note: Shepherds of people had need know the calendars of
         tempests of state. --Bacon.

   {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}.

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.

   {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.

Source : WordNet®

Gregorian calendar
     n : 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: {New Style calendar}]