The Collaborative International Dictionary of English v.0.48:

Decapitation \De*cap`i*ta"tion\, n. [LL. decapitatio: cf. F.
   d['e]capitation.]
   The act of beheading; beheading.
   [1913 Webster]

WordNet (r) 3.0 (2006):

decapitation
    n 1: execution by cutting off the victim's head [syn:
         decapitation, beheading]
    2: killing by cutting off the head [syn: decapitation,
       beheading]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

DECAPITATION, punishment. The punishment of putting a person to death by
taking off his head.