The Collaborative International Dictionary of English v.0.48:

Requiem \Re"qui*em\ (r?"kw?-?m;277), n. [Acc. of L. requies
   rest, the first words of the Mass being "Requiem aeternam
   dona eis, Domine," give eternal rest to them, O lord; pref.
   re- re + quies quiet. See Quiet, n., and cf. Requin.]
   1. (R.C.Ch.) A mass said or sung for the repose of a departed
      soul.
      [1913 Webster]

            We should profane the service of the dead
            To sing a requiem and such rest to her
            As to peace-parted souls.             --Shak.
      [1913 Webster]

   2. Any grand musical composition, performed in honor of a
      deceased person.
      [1913 Webster]

   3. Rest; quiet; peace. [Obs.]
      [1913 Webster]

            Else had I an eternal requiem kept,
            And in the arms of peace forever slept. --Sandys.
      [1913 Webster]

WordNet (r) 3.0 (2006):

requiem
    n 1: a song or hymn of mourning composed or performed as a
         memorial to a dead person [syn: dirge, coronach,
         lament, requiem, threnody]
    2: a musical setting for a Mass celebrating the dead
    3: a Mass celebrated for the dead

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "requiem":
   Dry Mass, Eucharistic rites, Lady Mass, Low Mass, Mass, Missa,
   Missa bassa, Missa cantata, Missa legata, Missa media,
   Missa praesanctificatorum, Missa privata, Missa publica,
   Missa sicca, Negro spiritual, Requiem Mass, Rosary, Rosary Mass,
   anthem, burial service, cantata, canticle, chorale, church music,
   coronach, dead march, death knell, death song, deathwatch, dirge,
   doxology, elegy, epicedium, eulogy, exequies, extreme unction,
   funeral march, funeral oration, funeral rites, funeral song,
   gospel, gospel music, graveside oration, hymn, hymn-tune, hymnody,
   hymnology, introit, keen, knell, last duty, last honors,
   last offices, last rites, mass, monody, motet, muffled drums,
   obsequies, offertory, offertory sentence, oratorio, paean,
   passing bell, passion, prosodion, psalm, psalmody, recessional,
   requiem mass, sacred music, spiritual, the Divine Liturgy,
   the Liturgy, threnode, threnody, viaticum, wake, white spiritual



The Devil's Dictionary (1881-1906):

REQUIEM, n.  A mass for the dead which the minor poets assure us the
winds sing o'er the graves of their favorites.  Sometimes, by way of
providing a varied entertainment, they sing a dirge.