Source : Webster's Revised Unabridged Dictionary (1913)

Regicide \Reg"i*cide\ (r?j"?*s?d), n. [F. r['e]gicide; L. rex,
   regis, a king + caedere to kill. Cf. {Homicide}.]
   1. One who kills or who murders a king; specifically
      (Eng.Hist.), one of the judges who condemned Charles I. to
      death.

   2. The killing or the murder of a king.

Source : WordNet®

regicide
     n 1: someone who commits regicide; the killer of a king
     2: the act of killing a king