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

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

ASSASSINATION, crim. law. A murder committed by an assassin. By
assassination is understood a murder committed for hire in money, without
any provocation or cause of resentment given by the person against whom the
crime is directed. Ersk. Inst. B. 4, t. 4, n. 45.