"Amen" is a Hebrew word meaning "truly" or "so be it," used throughout the Bible as an affirmation; Revelation 3 also uses it as a title for Christ.
Revelation 3:14 in the King James Version reads:
And unto the angel of the church of the Laodiceans write; These things saith the Amen, the faithful and true witness, the beginning of the creation of God;
The verse opens the letter to the church at Laodicea in Revelation 3. Read it in context at Revelation 3:14, or the full chapter at Revelation 3. Encyclopedia entry: en.wikipedia.org/wiki/Amen.