Regicide is the killing of a king (or queen). The word derives from the Latin regis, meaning "king," and the ancient French cide, meaning "killer." Today, the word regicide can also be applied to politicians who topple a president or prime minister.
People had been killing kings and queens (think Cleopatra) long before the term regicide really took off, which was after the execution of King Charles I in England in 1649. He was executed by guillotine, but a far worse fate awaited those who had signed off on his killing: they were hung, drawn, and quartered. Dangerous profession, regicide.