The Collaborative International Dictionary of English v.0.48:

Assassination \As*sas`si*na"tion\, n.
   The act of assassinating; a killing by treacherous violence.
   [1913 Webster]

WordNet (r) 3.0 (2006):

assassination
    n 1: an attack intended to ruin someone's reputation [syn:
         character assassination, assassination, blackwash]
    2: murder of a public figure by surprise attack