The Collaborative International Dictionary of English v.0.48:

Prosecutor \Pros"e*cu`tor\, n. [Cf. L. prosecutor an attendant.]
   1. One who prosecutes or carries on any purpose, plan, or
      business.
      [1913 Webster]

   2. (Law) The person who institutes and carries on a criminal
      suit against another in the name of the government.
      --Blackstone.
      [1913 Webster]

WordNet (r) 3.0 (2006):

prosecutor
    n 1: a government official who conducts criminal prosecutions on
         behalf of the state [syn: prosecutor, public
         prosecutor, prosecuting officer, prosecuting attorney]