The Collaborative International Dictionary of English v.0.48:

Platonic \Pla*ton"ic\, Platonical \Pla*ton"ic*al\, a. [L.
   Platonicus, Gr. ?: cf. F. platonique.]
   1. Of or pertaining to Plato, or his philosophy, school, or
      opinions.
      [1913 Webster]

   2. Pure, passionless; nonsexual; philosophical.
      [1913 Webster]

   Platonic bodies, the five regular geometrical solids;
      namely, the tetrahedron, hexahedron or cube, octahedron,
      dodecahedron, and icosahedron.

   Platonic love, a pure, spiritual affection, subsisting
      between persons of opposite sex, unmixed with carnal
      desires, and regarding the mind only and its excellences;
      -- a species of love for which Plato was a warm advocate.


   Platonic year (Astron.), a period of time determined by the
      revolution of the equinoxes, or the space of time in which
      the stars and constellations return to their former places
      in respect to the equinoxes; -- called also great year.
      This revolution, which is caused by the precession of the
      equinoxes, is accomplished in about 26,000 years.
      --Barlow.
      [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

WordNet (r) 3.0 (2006):

Platonic year
    n 1: time required for one complete cycle of the precession of
         the equinoxes, about 25,800 years [syn: great year,
         Platonic year]