The Collaborative International Dictionary of English v.0.48:

Assassin \As*sas"sin\, n. [F. (cf. It. assassino), fr. Ar.
   `hashishin one who has drunk of the hashish. Under its
   influence the Assassins of the East, followers of the Shaikh
   al-Jabal (Old Man of the Mountain), were said to commit the
   murders required by their chief.]
   One who kills, or attempts to kill, by surprise or secret
   assault; one who treacherously murders any one unprepared for
   defense.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Assassin \As*sas"sin\, v. t.
   To assassinate. [Obs.] --Stillingfleet.
   [1913 Webster]

WordNet (r) 3.0 (2006):

assassin
    n 1: a murderer (especially one who kills a prominent political
         figure) who kills by a surprise attack and often is hired
         to do the deed; "his assassins were hunted down like
         animals"; "assassinators of kings and emperors" [syn:
         assassin, assassinator, bravo]
    2: a member of a secret order of Muslims (founded in the 12th
       century) who terrorized and killed Christian Crusaders

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "assassin":
   Cain, alarmist, apache, assassinator, bloodletter, bloodshedder,
   bomber, bravo, burker, butcher, button man, cannibal, cutthroat,
   desperado, eradicator, executioner, exterminator, garroter,
   gorilla, gun, gunman, gunsel, hatchet man, head-hunter, hit man,
   homicidal maniac, homicide, killer, man-eater, man-killer,
   manslayer, massacrer, matador, murderer, pesticide, poison,
   poisoner, scaremonger, slaughterer, slayer, strangler, terrorist,
   thug, torpedo, trigger man



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

ASSASSIN, crim, law. An assassin is one who attacks another either
traitorously, or with the advantage of arms or place) or of a number of
persons who support him, and kills his victim. This being done with malice,
aforethought, is murder. The term assassin is but little used in the common
law, it is borrowed from the civil law.