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

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.