Source : Webster's Revised Unabridged Dictionary (1913)

Madrigal \Mad"ri*gal\, n. [It. madrigale, OIt. madriale,
   mandriale (cf. LL. matriale); of uncertain origin, possibly
   fr. It mandra flock, L. mandra stall, herd of cattle, Gr. ?
   fold, stable; hence, madrigal, originally, a pastoral song.]
   1. A little amorous poem, sometimes called a {pastoral poem},
      containing some tender and delicate, though simple,
      thought.

            Whose artful strains have oft delayed The huddling
            brook to hear his madrigal.           --Milton.

   2. (Mus.) An unaccompanied polyphonic song, in four, five, or
      more parts, set to secular words, but full of counterpoint
      and imitation, and adhering to the old church modes.
      Unlike the freer glee, it is best sung with several voices
      on a part. See {Glee}.

Source : WordNet®

madrigal
     n : an unaccompanied partsong for 2 or 3 voices; follows a
         strict poetic form
     v : sing madrigals; "The group was madrigaling beautifully"