A republic is a form of government in which power resides in the people, and the government is ruled by elected leaders run according to law (from Latin: res publica), rather than inherited or appointed (such as through inheritance or divine mandate). In modern times, the definition of a republic is also commonly limited to a government which excludes a monarch. Currently, 135 of the world's 206 sovereign states use the word "republic" as part of their official names.Both modern and ancient republics vary widely in their ideology and composition. In the classical and medieval period of Europe, many states were fashioned on the Roman Republic, which referred to the governance of the city of Rome, between it having kings and emperors. The Italian medieval and Renaissance political tradition today referred to as "civic humanism", is sometimes considered to derive directly from Roman republicans such as Sallust and Tacitus. However, Greek-influenced Roman authors, such as Polybius and Cicero, sometimes also used the term as a translation for the Greek politeia which could mean regime generally, but could also be applied to certain specific types of regime which did not exactly correspond to that of the Roman Republic. Republics were not equated with classical democracies such as Athens, but had a democratic aspect.Republics became more common in the Western world starting in the early 19th century, eventually displacing absolute monarchy as the most common form of government. In modern republics the executive is legitimized both by a constitution and by popular suffrage. Montesquieu included both democracies, where all the people have a share in rule, and aristocracies or oligarchies, where only some of the people rule, as republican forms of government.Most often a republic is a sovereign state, but there are also sub-sovereign state entities that are referred to as republics, or which have governments that are described as 'republican' in nature. For instance, Article IV of the United States Constitution "guarantee[s] to every State in this Union a Republican form of Government". Similarly, the Soviet Union was constitutionally described as a "federal multinational state", composed of 15 republics, two of which – Ukraine and Belarus – had their own seats at the United Nations.