The word amen is a declaration of affirmation found in the Hebrew Bible

Hebrew Bible

The Hebrew Bible is a term used by biblical scholars outside of Judaism to refer to the Tanakh , a canonical collection of Jewish texts, and the common textual antecedent of the several canonical editions of the Christian Old Testament...

 and New Testament

New Testament

The New Testament is the second major division of the Christian biblical canon, the first such division being the much longer Old Testament....

. Its use in Judaism

Judaism

Judaism ) is the "religion, philosophy, and way of life" of the Jewish people...

 dates back to its earliest texts. It has been generally adopted in Christian

Christianity

Christianity is a monotheistic religion based on the life and teachings of Jesus as presented in canonical gospels and other New Testament writings...

 worship as a concluding word for prayer

Prayer

Prayer is a form of religious practice that seeks to activate a volitional rapport to a deity through deliberate practice. Prayer may be either individual or communal and take place in public or in private. It may involve the use of words or song. When language is used, prayer may take the form of...

s and hymn

Hymn

A hymn is a type of song, usually religious, specifically written for the purpose of praise, adoration or prayer, and typically addressed to a deity or deities, or to a prominent figure or personification...

s. In Islam

Islam

Islam . The most common are and .   : Arabic pronunciation varies regionally. The first vowel ranges from ~~. The second vowel ranges from ~~~...

, it is the standard ending to Dua

Dua

In the terminology of Islam, Duʿāʾ Plural: Dow'āt literally meaning invocation, is an act of supplication. The term is derived from an Arabic word meaning to 'call out' or to 'summon', and Muslims regard this as a profound act of worship. This is when Muslim people from all over connect with God...

 (supplication) and the "Opening

Al-Fatiha

Sura Al-Fatiha , is the first chapter of the Qur'an. Its seven verses are a prayer for Allah's guidance, and stress His Lordship and Mercy...

" chapter of the Qur'an

Qur'an

The Quran , also transliterated Qur'an, Koran, Alcoran, Qur’ān, Coran, Kuran, and al-Qur’ān, is the central religious text of Islam, which Muslims consider the verbatim word of God . It is regarded widely as the finest piece of literature in the Arabic language...

. Common English translations of the word amen include "verily" and "truly". It can also be used colloquially to express strong agreement, as in, for instance, amen to that.
In English, the word amen has two primary pronunciations, ah-men (/ɑːˈmɛn/) or ay-men (/eɪˈmɛn/), with minor additional variation in emphasis (the two syllables may be equally stressed instead of placing primary stress on the second).