Source : Webster's Revised Unabridged Dictionary (1913)

Te Deum \Te De"um\
   A musical setting of the Te Deum.

Te Deum \Te` De"um\ [L., from te (accus. of tu thou) + Deum,
   accus. of Deus God. See {Thou}, and {Deity}.]
   1. An ancient and celebrated Christian hymn, of uncertain
      authorship, but often ascribed to St. Ambrose; -- so
      called from the first words ``Te Deum laudamus.'' It forms
      part of the daily matins of the Roman Catholic breviary,
      and is sung on all occasions of thanksgiving. In its
      English form, commencing with words, ``We praise thee, O
      God,'' it forms a part of the regular morning service of
      the Church of England and the Protestant Episcopal Church
      in America.

   2. A religious service in which the singing of the hymn forms
      a principal part.

Source : WordNet®

Te Deum
     n : an ancient liturgical hymn