Derived from the Latin expression res publica (“the public thing”), the category of republic could encompass not only democratic states but also oligarchies, aristocracies, and monarchies. Prior to the 17th century, the term was used to designate any state, with the exception of tyrannical regimes, because their object is not the common good but the private benefit of a single individual. During the 17th and 18th centuries, the meaning of republic shifted with the growing resistance to absolutist regimes, and it eventually came to designate a form of government in which the leader is periodically appointed under a constitution, in contrast to hereditary monarchies.

Source: Britannica