This word is used in Judaism and Christianity as an ending word for prayers and hymns. When saying "amen," worshippers confirm their belief in God's word, or affirm agreement with what is being preached or recited.
In Islam, the pronuciation "ameen" is often recited during daily prayers at the end of Surah Al-Fatihah (the first chapter of the Qur'an). It is also said during and at the end of personal supplications (du'a).
Any use of ameen in Islamic prayer is considered optional (sunnah), not required (wajib). The practice is based on the example and teachings of the Prophet Muhammad, peace be upon him. He reportedly told his followers to say 'ameen' after the imam (prayer leader) finishes reciting the Fatiha because: "If a person's saying 'ameen' at that time coincides with the angels saying 'ameen', his previous sins will be forgiven."
There is some difference of opinion among Muslims about whether "ameen" should be said during prayer in a quiet voice or a loud voice. Very few debate about whether it should be said at all; its use is widespread among Muslims.