The Collaborative International Dictionary of English v.0.48:

Ablative \Ab"la*tive\, (Gram.)
   The ablative case.
   [1913 Webster]

   ablative absolute, a construction in Latin, in which a noun
      in the ablative case has a participle (either expressed or
      implied), agreeing with it in gender, number, and case,
      both words forming a clause by themselves and being
      unconnected, grammatically, with the rest of the sentence;
      as, Tarquinio regnante, Pythagoras venit, i. e.,
      Tarquinius reigning, Pythagoras came.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ablative absolute
    n 1: a constituent in Latin grammar; a noun and its modifier can
         function as a sentence modifier