Definition:

The Peace of Nicias was named for an Athenian leader, Nicias, who came to power after the death of the great Athenian leader Pericles (who led the people of Attica behind the city walls at the start of the Peloponnesian War where in the congested conditions, he died of plague), and opposed, but then led the Sicilian Expedition. Just as Pericles was associated with the Peloponnesian War, so Nicias was associated with the peace and it was for this reason, according to Plutarch, that it was given his name.

The Peace of Nicias was a treaty concluded in 421 B.C. between Sparta and Athens, during the Peloponnesian War. It was supposed to last for 50 years and cover the allies of each side, basically returning matters to their status before the war.

Allies were not consulted, and Spartan allies were unwilling to abide by the treaty.

Within seven years the Peace of Nicias was over and the Ionian War stage of the Peloponnesian War began with the Spartans breaking the treaty by occupying the Attic village of Declea.

Examples: About the Peace of Nicias, Plutarch writes:

The articles being, that the garrisons and towns taken on either side, and the prisoners should be restored, and they to restore the first to whom it should fall by lot, Nicias, as Theophrastus tells us, by a sum of money procured that the lot should fall for the Lacedaemonians to deliver the first.
Plutarch's Life of Nicias