The concept of 'ancient democracy' requires careful qualification: the Athenian democracy that classical scholars study was one specific experiment, not a general ancient phenomenon, and even within the Greek world it was surrounded by non-democratic political forms.

The distribution of political forms across the Greek city-states was remarkably varied: democracy at Athens (and several other cities in its sphere of influence), oligarchy in most other poleis, hereditary monarchy in Macedon and Sparta's dual kingship (combined with ephorate and gerousia in ways that defy simple categorization), and tyrannies (one-man rules typically installed by a charismatic military leader) that arose repeatedly and were widely condemned even when they governed well.

The Greek political vocabulary that we have inherited — democracy, oligarchy, tyranny, monarchy, aristocracy, polity — comes from Greek theorists (primarily Aristotle) who were doing comparative political science on these actually existing variations. Aristotle's classification in the Politics distinguishes constitutions by the number of rulers (one, few, many) and by whether they rule for the common good (correct forms) or for their own interest (deviant forms), producing the famous sixfold table.

The Roman republic presents a different model: a mixed constitution in which elements of monarchy (consuls), aristocracy (Senate), and democracy (popular assemblies) balanced each other. Polybius, writing in the 2nd century BC, held that this balance was the secret of Roman success. The question of whether this balance was genuine or whether the Senate's informal supremacy made it effectively an aristocracy despite its constitutional complexity was debated in antiquity and remains productive for comparative constitutional analysis.