From The Art and Popular Culture Encyclopedia
Ostracism (ὀστρακισμός, ostrakismos) was a procedure under the Athenian democracy in which any citizen could be expelled from the city-state of Athens for ten years. While some instances clearly expressed popular anger at the citizen, ostracism was often used preemptively. It was used as a way of neutralizing someone thought to be a threat to the state or potential tyrant.
Crucially, ostracism had no relation to the processes of justice. There was no charge or defense, and the exile was not in fact a penalty; it was simply a command from the Athenian people that one of their number be gone for ten years.
A modern use developed from the term is to describe informal exclusion from a group through social rejection. Although the psychology of ostracism takes this further, where it has been defined as “...any behaviour in which a group or individual excludes and ignores another group or individual”. This could therefore be an intentional act or an unintentional one.