Reading Roman history, but not rightly, early modern Western political theorists divided governments into two basic categories, monarchies and republics, defining the latter as self-governing polities without a monarch and understanding the former as either absolute or constitutional.
As Kaldellis explains, the ancient Greeks and Romans saw things differently.
Their two basic categories were kingdoms and commonwealths. A kingdom, in their experience, was the possession of a king ruled by his might for his own satisfaction. A commonwealth—res publica in Latin, politeia in Greek—was an independent polity variously governed but administered for the good of all.