Word found at the end of prayers has a long, complex history.
Reprinted from The Jewish Religion: A Companion, published by Oxford University Press.
Amen is the liturgical response now used not only in Judaism but also in Christianity and Islam. The word has the same Hebrew root as emunah (faith) and is also connected with the word emet meaning "truth." The idea expressed is of firm trust, acceptance, and reliability.
Amen is found in a variety of contexts in the Bible (Numbers 5: 22; Deuteronomy 27: 15; 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26; I Kings I: 36; Isaiah 65: 16; Jeremiah II: 5; 28: 6; I Chronicles 16: 36; Nehemiah 5: 13; 8: 6; Psalms 41: 14; 72: 19; 89: 52; 106: 48). Louis Ginzberg has translated amen as "So be it" or "So shall it be" and has described it as "perhaps the most widely known word in human speech."
Defining the Word
A late second-century teacher in the Talmud takes the initial letters of amen to represent el melekh neeman, "God, Faithful King." A later Jewish commentator to the prayer book interprets homiletically the initial letters as: ani moser nafshi, "I offer up myself as a sacrifice." A rabbinic saying has it that one who responds amen to a benediction is greater than the one who recites the benediction. The reason given for this statement by the medieval sages of England is that the one who responds with amen also hears the benediction itself and, since "to hear is akin to pronouncing," he has to his credit both the amen and the benediction.
It may also be that the statement is intended to express the thought that it is more praiseworthy, because more difficult, to give assent to a truth first seen by others than to be a pioneer in discovering the truth for oneself. On the other hand, it is said that amen should not be recited in a louder voice than that of the one who recites the benediction, perhaps because this would imply a "holier than thou" attitude.
A saying attributed to the second-century teacher Rabbi Meir has it that a child merits the World to Come from the day it first says amen. Another Rabbinic saying is that all the gates of heaven open to one who recites amen with all his strength, explained by the great French commentator Rashi as meaning with all his powers of concentration.
The Rules about Amen
The rules in the codes regarding the response amen are that it must not be "orphaned" from the benediction to which it is the response by coming too soon or too late; it should not be slurred but perfectly distinct; and it should only be recited after a benediction pronounced by someone else, not after one's own benediction. When said in response to a prayer of petition the intention should be: "May it be Thy will that this purpose be realized."