In the Athenian democracy ostracism was not a term describing social exclusion but a legal process of the democracy under which a single citizen was exiled for a 10 year period without other loss of rights.

The word ostracism comes from the Greek word ostracon (ὄστρακον) meaning pottery fragment or potsherd. In a world short of papyrus (an expensive import from Egypt) casual sketches, note-taking, and balloting were performed on fragments of pottery. 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 10 years on penalty of death if he remained in Attica. 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 the ostracized to deal with special circumstances. For instance, Aristides returned to the service of Athens in the recall during the Persian Wars and materially aided the state at the Battle of Salamis.

To be added:

Aristides and the ostracism anecdote
evidence for potential ostracism fraud

Some citizens known to have been ostracized:

Aristides, 480s
Cimon, late 460s
Themistocles, late 470s