In modern parlance, to ostracize means to exclude someone from society or from a community, by not communicating with or even noticing them, similar to shunning. In a historical context, the term refers to ostracism, which was a legal process during the Athenian democratic period in the Ancient Greek city-state of Athens. Under this procedure, a single citizen was exiled for a ten-year period without other loss of rights.

The word "ostracism" comes from the Greek word ostrakon (ὄστρακον) meaning "pottery fragment" or "potshard."

Because papyrus was an expensive import from Egypt, the ancient Greeks used fragments of pottery for casual sketches, note-taking, and voting. In January of each year, the assembly took a vote on ostracism. If at the designated assembly meeting there were at least 6,000 ballots cast, whichever citizen received a simple majority of the votes was exiled for ten years, under which he could not return to Attica under penalty of death. The citizen lost the right to participate in politics by virtue of his absence, but his property was not confiscated, and he could appoint a manager to deal with his affairs and forward any income.

The minimum figure of 6,000 ballots required has been interpreted by historians to mean that an attendance of 6,000 citizens at an assembly may have been a high number, rather than a regularly expected monthly number.

Many well-known politicians of democracy were ostracized at one time or another, and periodically the democracy passed special laws recalling an ostracized person to deal with special circumstances. Aristides returned to the service of Athens in the recall during the Persian Wars and materially aided the state at the Battle of Salamis.

Evidence exists that there was electoral fraud on occasion. Ostraca have been found with the same name on them, obviously written by the same person (all of them had the same misspelling). The analysis of this evidence is that the pre-written ostraca were designed to be handed out to random citizens to help oust a certain person.

Notable individuals that were ostracized include Aristides (480s BC), Cimon (late 460s BC), and Themistocles (late 470s BC).