The right of return is a principle which is drawn from the Universal Declaration of Human Rights and the International Covenant on Civil and Political Rights, intended to enable people to return to, and re-enter, their country of origin.
The 1951 Convention relating to the Status of Refugees (read together with its 1967 Protocol) does not give refugees a right to return, but rather prohibits return (refoulment) to a country where he or she faces serious threats to his or her life or freedom. The Convention binds the many countries which have ratified it.
By contrast, the right of return has not passed into customary international law, although it remains an important aspirational human right. Instead, international law gives each country the right to decide for itself to whom it will give citizenship.
There are various recorded cases in ancient history where people deported or uprooted from their city or homeland were allowed (or encouraged) to return, typically when the balance of military and political forces which caused their exile had changed.
A well-known example is the Return to Zion, when King Cyrus the Great allowed the Jews expelled in the Babylonian Exile to return and rebuild Jerusalem. Recorded in the Hebrew Bible (Book of Ezra and Book of Nehemiah) this case is often cited as a precedent by modern Zionism and also inspired other groups seeking to pursue their own claim for a Right of Return.
The wars of Classical Greece provide several more examples. In the Peloponesian War, Athenians destroyed the island city of Melos, killed all adult male citizens which they caught and sold the women and children into slavery. Following Athens' defeat, in 405 BC, the Spartan general Lysander expelled the Athenian settlers from Melos and restored the survivors of the original Melians to the island. The victorious Spartans and their allies made an effort to trace and liberate the dispersed Melians and restore them to their homeland.