Ostraca (singular ostracon) is from the Greek word 'ostrakon' meaning 'shell.' Archaeologists use this term to refer to ceramic pot sherds with inscriptions on them, the inscriptions placed after the pot was broken and thus separate from normal decoration.
Ostraca were used in many of the classic civilizations, leading to artifacts in Greek, Roman, Egyptian, and Hebrew writing systems. The English word 'ostracism' is related, because the Greeks used ostraca to vote out misbehaving residents; but ostraca were also used for tax receipts, legal documents, and miscellaneous note-taking. As such, they are small but extremely important types of artifacts.
This glossary entry is a part of the About.com Dictionary of Archaeology.