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