The Collaborative International Dictionary of English v.0.48:

Executioner \Ex`e*cu"tion*er\, n.
   1. One who executes; an executer. --Bacon.
      [1913 Webster]

   2. One who puts to death in conformity to legal warrant, as a
      hangman.
      [1913 Webster]

WordNet (r) 3.0 (2006):

executioner
    n 1: an official who inflicts capital punishment in pursuit of a
         warrant [syn: executioner, public executioner]

Easton's 1897 Bible Dictionary:

Executioner
   (Mark 6:27). Instead of the Greek word, Mark here uses a Latin
   word, speculator, which literally means "a scout," "a spy," and
   at length came to denote one of the armed bodyguard of the
   emperor. Herod Antipas, in imitation of the emperor, had in
   attendance on him a company of speculatores. They were sometimes
   employed as executioners, but this was a mere accident of their
   office. (See MARK, GOSPEL OF.)


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

EXECUTIONER. The name given to him who puts criminals to death, according to
their sentence; a hangman.
     2. In the United States, executions are so rare that there are no
executioners by profession. It is the duty of the sheriff or marshal to
perform this office, or to procure a deputy to do it for him.