NOUN (2)

1. someone who commits regicide; the killer of a king;

2. the act of killing a king;

The Collaborative International Dictionary of English v.0.48:

Regicide \Reg"i*cide\ (r[e^]j"[i^]*s[imac]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.
      [1913 Webster]

   2. The killing or the murder of a king.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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