The Collaborative International Dictionary of English v.0.48:

Duel \Du"el\, n. [It. duello, fr. L. duellum, orig., a contest
   between two, which passed into the common form bellum war,
   fr. duo two: cf. F. duel. See Bellicose, Two, and cf.
   Duello.]
   A combat between two persons, fought with deadly weapons, by
   agreement. It usually arises from an injury done or an
   affront given by one to the other.
   [1913 Webster]

   Trial by duel (Old Law), a combat between two persons for
      proving a cause; trial by battel.
      [1913 Webster]