The definitions of the republics, especially historical ones, were highly varied, although in the period we're speaking about, the term 'republic' was generally understood as the country that is not a patrimonial monarchy, i.e. one that does not belong to a single monarch or a royal family and is governed by various other powers than the nominal ruler, usually with aristocracy or nobility being the most prominent group, with self-governed cities representing local sources of power. Both Nicolo Machiavelli and later also Montesqieu defined republics as the states governed solely or to large extent by a diverse groups participating in the policy-making and administration. Similarly, Jean Bodin stressed that a republic is a country, where various groups work for a common goal (i.e. country that is not a sole property of the king), stating that system that we will call today an absolutist monarchy is the best example of a republic. But in general, any country that was not a patrimonial monarchy and was characterized by a significant degree of participation of other social groups than the royal family and officials designated by the latter in the central governance could have been considered a republic by early modern politicians, philosophers or other people well-versed in contemporary politics.

These definitions are different from is understood as a modern republic, that is a country that is in no way a property of a single representative of the government and where the government is not fixed, usually being elected by a popular vote on all levels. Such model of a republican country emerged only in late 18th century, with the success of the American War of Independence and formation of the United States that was a republic with a democratically elected government.

The country known in English historiography as the Polish-Lithuanian Commonwealth was usually called simply 'The Republic' by the indigenous Poles, Lithuanians and Ruthenians, and although this designation was not formal, it was on occasion called 'The Polish-Lithuanian Republic' or 'The Most Serene Polish Republic' (Serenissima Res Publica Poloniae) in an obvious nod to an informal name of the Venetian Republic (although in the Reciprocal Guarantee of Two Nations signed in 1791 the name 'Republic of Two Nations' has been used in an official capacity). The country itself, although nominally known as Kingdom of Poland and Grand Duchy of Lithuania could have easily been considered a republic by the contemporary scholars given the high level of political participation of the nobility. The position of a monarch was relatively weak (or very weak in comparison with absolutist monarchies of 17th century), as expressed in the maxim attributed to Chancellor Jan Zamoyski 'rex regnat et non gubernat' (king reigns and does not govern). In essence, the king was the head of executive branch of government with the prerogative not dissimilar from modern presidents (and since 1573 he was also elected to boot). In addition, the nobility could have legally rebelled against the king who have broken the law, making the latter accountable. The main political body responsible for the bulk of legislative actions was the recurrent General Convocation, with the local laws being in the competences of the local parliaments. Additionally, nobility was able to create confederacies, i.e. the temporary organizations attempting to achieve specific political goals.

Thus, such a large enfranchisement of the nobility that in the 17th century accounted for roughly 10-11% of the entire population of the country, complete with the electional model of monarchy, where the loyalty of royal subjects (at least the aristocracy and nobility) was not unconditional and the larger part of laws being presented and accepted during parliamentary sessions by the representatives of the respective administrative regions was definitely enough to call the country a republic by its contemporaries.