The Collaborative International Dictionary of English v.0.48:

Suffocation \Suf`fo*ca"tion\, n. [L. suffocatio: cf. F.
   suffocation.]
   The act of suffocating, or the state of being suffocated;
   death caused by smothering or choking.
   [1913 Webster]

   Note: The term suffocation is sometimes employed synonymously
         with asphyxia. In the strict medico-legal sense it
         signifies asphyxia induced by obstruction of the
         respiration otherwise than by direct pressure on the
         neck (hanging, strangulation) or submersion (drowning).
         --Quain.
         [1913 Webster]

WordNet (r) 3.0 (2006):

suffocation
    n 1: killing by depriving of oxygen [syn: suffocation,
         asphyxiation]
    2: the condition of being deprived of oxygen (as by having
       breathing stopped); "asphyxiation is sometimes used as a form
       of torture" [syn: suffocation, asphyxiation]