ostraca

/AH0 S T R AA1 K AH0/
noun
  1. 1

    A piece of pottery or stone, usually broken off from a vase or other earthenware vessel, especially one used to cast a vote during the Ancient Greek process of ostracism.

Source: https://en.wiktionary.org/wiki/ostraca, https://en.wiktionary.org/wiki/ostracon