From The Collaborative International Dictionary of English v.0.48:

Millennium \Mil*len"ni*um\ (m[i^]l*l[e^]n"n[i^]*[u^]m), n. [LL.,
   fr. L. mille a thousand + annus a year. See Mile, and
   Annual.]
   1. A period of one thousand years.
      [1913 Webster]

   2. Specifically: The period of a thousand years mentioned in
      the twentieth chapter of Revelation, during which holiness
      is to be triumphant throughout the world. Some believe
      that, during this period, Christ will reign on earth in
      person with his saints.
      [1913 Webster]

   3. Hence: A long period of happiness, righteousness, and
      prosperity, usually considered as being in the indefinite
      future.
      [PJC]

   4. A thousandth anniversary; especially, Each first day of
      January falling in a year which is a multiple of one
      thousand, such as in 1000 a. d. or 2000 a. d.; as, the
      second millenium will be celebrated on January 1, 2000;
      also used attributively, as a millenium celebration.

   Note: Technically, if the calendar of the Common Era (Anno
         Domini) is considered as beginning on January 1, 1 a.
         d., then the millenium will fall in each year ending in
         001, as in 1001 a. d. or January 1, 2001 a. d.. However
         in the common culture, the change of the first digit of
         the year from 1 to 2, as from 1999 to 2000 is
         considered as the more symbolic event, especially since
         the dating of the beginning of the Christian era is
         somewhat arbitrary, having been an attempt to fix the
         date of the birth of Christ, and being considered by
         scholars as being in error by as much as five years.
         [PJC]

From WordNet (r) 2.0:

millennium
     n 1: a span of 1000 years
     2: (New Testament) in Revelations it is foretold that those
        faithful to Jesus will reign with Jesus over the earth for
        a thousand years; the meaning of these words have been
        much debated; some denominations (e.g. Jehovah's
        Witnesses) expect it to be a thousand years of justice and
        peace and happiness
     3: the 1000th anniversary (or the celebration of it)
     [also: millennia (pl)]

From Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "millennium":
   Agapemone, Arcadia, Big Rock-Candy Mountain, Canaan,
   Cloudcuckooland, Cockaigne, Eden, Eldorado, Erewhon, G,
   Garden of Eden, Goshen, Happy Valley, Land of Youth, Laputa, M,
   Never-Never-land, Neverland, New Atlantis, Pandemonium, Quivira,
   Saturnia regna, Saturnian age, Shangri-la, Utopia, abundant year,
   academic year, age of Aquarius, annum, bissextile year,
   calendar month, calendar year, century, chiliad, chiliagon,
   chiliahedron, chiliarch, chiliarchia, cloudland, common year, day,
   decade, decennary, decennium, defective year, dreamland, dystopia,
   era of prosperity, faerie, fair weather, fairyland, fiscal year,
   fortnight, golden age, golden era, golden time, good times, grand,
   halcyon days, heaven, heyday, hour, kakotopia, kilo, kilocycle,
   kilogram, kilohertz, kiloliter, kilometer, kingdom come, lakh,
   land of dreams, land of enchantment, land of faerie,
   land of plenty, land of promise, leap year, lotus land,
   lunar month, lunar year, lunation, luster, lustrum, man-hour,
   microsecond, millepede, milligram, milliliter, millisecond, minute,
   moment, month, moon, myriad, one hundred thousand, palmy days,
   paradise, piping times, promised land, prosperity, quarter,
   quinquennium, regular year, reign of Saturn, rosy era, second,
   semester, session, sidereal year, solar year, sun, sunshine,
   ten thousand, term, thou, thousand, trimester, twelvemonth, utopia,
   week, weekday, wonderland, yard, year

From Easton's 1897 Bible Dictionary:

Millennium
   a thousand years; the name given to the era mentioned in Rev.
   20:1-7. Some maintain that Christ will personally appear on
   earth for the purpose of establishing his kingdom at the
   beginning of this millennium. Those holding this view are
   usually called "millenarians." On the other hand, it is
   maintained, more in accordance with the teaching of Scripture,
   we think, that Christ's second advent will not be premillennial,
   and that the right conception of the prospects and destiny of
   his kingdom is that which is taught, e.g., in the parables of
   the leaven and the mustard-seed. The triumph of the gospel, it
   is held, must be looked for by the wider and more efficient
   operation of the very forces that are now at work in extending
   the gospel; and that Christ will only come again at the close of
   this dispensation to judge the world at the "last day." The
   millennium will thus precede his coming.
   

From THE DEVIL'S DICTIONARY ((C)1911 Released April 15 1993):

MILLENNIUM, n.  The period of a thousand years when the lid is to be
screwed down, with all reformers on the under side.