Easton's Bible Dictionary (1897)
This Hebrew word means firm, and hence also faithful (Rev. 3:14). ” 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). , they are all true and sure (2 Cor. 1:20).
Smith's Bible Dictionary (1863)
literally “true” and used as a substantive, “that which is true,” “truth,” (Isaiah 65:16) a word used in strong asseverations, fixing, as it were, the stamp of truth upon the assertion which it accompanied, and making it binding as an oath. Comp. (Numbers 5:22) In the synagogues and private houses it was customary for the people or members of the family who were present to say “amen” to the prayers which were offered.
Hastings' Dictionary of the Bible (1898) & Schaff's Bible Dictionary
AMEN' (literally, true, firm: metaphorically, faithful), used to denote assent or entire acquiescence, impressing the stamp of absolute truthfulness upon the statement. Deut 27:15. It was used as the solemn affirmative response to an oath. The word was often repeated. " Of this a trace is supposed to remain in the concluding sentence of the Lord's Prayer. Comp. Rom 11:36.
But in the synagogue and private houses it was customary for the people or members of the family who were present to say "Amen" to the prayers which were offered by the minister or master of the house, and the custom remained in the early Christian Church. Matt 6:13; 1 Cor 14:16. " It is sometimes translated verily, and was frequently used by our Saviour when he was about to utter some distinct, important, and solemn truth. Its repetition, "Verily, verily, I say unto you" (in John) strengthens the assertion. The promises of God are amen, because they are made sure and certain in Christ.
2 Cor 1:20. Amen is one of the titles of our blessed Saviour, Rev 3:14, as he is the faithful and true witness.