Isopolity refers to the concept of "equal citizenship" in Ancient Greek city-states. It was an arrangement in which citizens of two independent cities were granted reciprocal rights.
Ancient Greece wasn't one country in the way that we think of a nation today. It was a collection of independent mini-nations, or city-states, such as Sparta, Athens, and Thebes. Isopolity was a system where citizens of one city-state were granted the same rights as those from another — such as the right to vote, hold political office, or own property — if they moved there. While there is no perfect modern equivalent, the European Union is similar in that citizens of member nations can move freely across borders and share certain civic rights.