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:

Year \Year\, n. [OE. yer, yeer, [yogh]er, AS. ge['a]r; akin to
   OFries. i?r, g?r, D. jaar, OHG. j[=a]r, G. jahr, Icel. [=a]r,
   Dan. aar, Sw. [*a]r, Goth. j?r, Gr. ? a season of the year,
   springtime, a part of the day, an hour, ? a year, Zend
   y[=a]re year. [root]4, 279. Cf. Hour, Yore.]
   [1913 Webster]
   1. The time of the apparent revolution of the sun trough the
      ecliptic; the period occupied by the earth in making its
      revolution around the sun, called the astronomical year;
      also, a period more or less nearly agreeing with this,
      adopted by various nations as a measure of time, and
      called the civil year; as, the common lunar year of 354
      days, still in use among the Mohammedans; the year of 360
      days, etc. In common usage, the year consists of 365 days,
      and every fourth year (called bissextile, or leap year) of
      366 days, a day being added to February on that year, on
      account of the excess above 365 days (see Bissextile).
      [1913 Webster]

            Of twenty year of age he was, I guess. --Chaucer.
      [1913 Webster]

   Note: The civil, or legal, year, in England, formerly
         commenced on the 25th of March. This practice continued
         throughout the British dominions till the year 1752.
         [1913 Webster]

   2. The time in which any planet completes a revolution about
      the sun; as, the year of Jupiter or of Saturn.
      [1913 Webster]

   3. pl. Age, or old age; as, a man in years. --Shak.
      [1913 Webster]

   Anomalistic year, the time of the earth's revolution from
      perihelion to perihelion again, which is 365 days, 6
      hours, 13 minutes, and 48 seconds.

   A year's mind (Eccl.), a commemoration of a deceased
      person, as by a Mass, a year after his death. Cf. A
      month's mind, under Month.

   Bissextile year. See Bissextile.

   Canicular year. See under Canicular.

   Civil year, the year adopted by any nation for the
      computation of time.

   Common lunar year, the period of 12 lunar months, or 354
      days.

   Common year, each year of 365 days, as distinguished from
      leap year.

   Embolismic year, or Intercalary lunar year, the period of
      13 lunar months, or 384 days.

   Fiscal year (Com.), the year by which accounts are
      reckoned, or the year between one annual time of
      settlement, or balancing of accounts, and another.

   Great year. See Platonic year, under Platonic.

   Gregorian year, Julian year. See under Gregorian, and
      Julian.

   Leap year. See Leap year, in the Vocabulary.

   Lunar astronomical year, the period of 12 lunar synodical
      months, or 354 days, 8 hours, 48 minutes, 36 seconds.

   Lunisolar year. See under Lunisolar.

   Periodical year. See Anomalistic year, above.

   Platonic year, Sabbatical year. See under Platonic, and
      Sabbatical.

   Sidereal year, the time in which the sun, departing from
      any fixed star, returns to the same. This is 365 days, 6
      hours, 9 minutes, and 9.3 seconds.

   Tropical year. See under Tropical.

   Year and a day (O. Eng. Law), a time to be allowed for an
      act or an event, in order that an entire year might be
      secured beyond all question. --Abbott.

   Year of grace, any year of the Christian era; Anno Domini;
      A. D. or a. d.
      [1913 Webster] year 2000 bug