Definitions: Amen is an exclamation expressing agreement with a statement. At the end of a prayer it expresses the genuineness of the petition.
Greek References: αμην (ameen), translated by Luke as αληθος (aleethos), truly (Mark 9:1, Luke 9:27).
Scripture:
Doxology and Benediction (praise and blessing)
Psalm 41:13 "Blessed be the Lord, the God of Israel, from everlasting and to everlasting! Amen and amen"
Matthew 6:13 "Yours is the Kingdom, the power, and the glory forever, Amen."
Revelation 22:20-21 "The grace of the Lord Jesus Christ be with all the saints. Amen"
Romans 11:36, 15:33 [typical doxology and benediction]
Solemn pronouncement
Nehemiah 5:13 Neh 8:6 "All the people answered, Amen, Amen, with the lifting up of their hands"
Romans 1:25, 9:1-5 [typical solemn affirmations]
Psalms 72:19; 89:52 [Endings to books of Psalms]
Thanksgiving
1Co 14:15-16 "...say the Amen at your giving of thanks..."
Comment: In the Bible, "Amen" is almost always the last word of a solemn statement such as a benediction or doxology. Jesus however would often use it at the beginning of a solemn statement by saying "Verily" or "Truly". This is actually the word "Amen".