A person who was previously a Muslim, either born a Muslim or became a Muslim then left the Islamic faith. Most ex-muslims are born into Islam, when they leave the do not bare witness that "There is no God but God(Allah in Arabic)), and Muhammad is the Messanger of God". In Islam you can be executed for apostacy in a state or Caliphate of empire which choses to emply the rules of Islam(known as the Sharia in Arabic) as the law of land, though it is very rare when people are executed for apostacy and really only happens if the apostate speaks about Islam in a very negative way using vulgar language and insulting figures held high by Muslims.