AMEN is a 4 letter word that starts with A.

1. \A`men"\ (?; 277), interj., adv., & n. [L. amen, Gr.
   'amh`n, Heb. [=a]m[=e]n certainly, truly.]
   An expression used at the end of prayers, and meaning, So be
   it. At the end of a creed, it is a solemn asseveration of
   belief. When it introduces a declaration, it is equivalent to
   truly, verily.
   Note: It is used as a noun, to denote:
   (a) concurrence in belief, or in a statement; assent;
   (b) the final word or act;
   (c) Christ as being one who is true and faithful.
             And let all the people say, Amen.    --Ps. cvi. 48.
             Amen, amen, I say to thee, except a man be born
             again, he can not see the kingdom of God. --John
                                                  ii. 3. Rhemish
                                                  Trans.
   {To say amen to}, to approve warmly; to concur in heartily or
      emphatically; to ratify; as, I say Amen to all.
   

2. \A`men"\, v. t.
   To say Amen to; to sanction fully.
   

This Hebrew word means firm, and hence also faithful (Rev. 3:14). In Isa. 65:16, the Authorized Version has "the God of truth," which in Hebrew is "the God of Amen." It is frequently used by our Saviour to give emphasis to his words, where it is translated "verily." Sometimes, only, however, in John's Gospel, it is repeated, "Verily, verily." It is used as an epithet of the Lord Jesus Christ (Rev. 3:14).

It is found singly and sometimes doubly at the end of prayers (Ps. 41:13; 72:19; 89:52), to confirm the words and invoke the fulfilment of them. It is used in token of being bound by an oath (Num. 5:22; Deut. 27:15-26; Neh. 5:13; 8:6; 1 Chr. 16:36). In the primitive churches it was common for the general audience to say "Amen" at the close of the prayer (1 Cor. 14:16).

The promises of God are Amen; i.e., they are all true and sure (2 Cor. 1:20).