1a) metaphorically faithful
2) verily, amen
2a) at the beginning of a discourse - surely, truly, of a truth
2b) at the end - so it is, so be it, may it be fulfilled. It was a custom, which passed over from the synagogues to the Christian assemblies, that when he who had read or discoursed, had offered up solemn prayer to God, the others responded Amen, and thus made the substance of what was uttered their own.
Part of Speech: particle indeclinable
Citing in TDNT: 1:335, 53
Of Hebrew origin [H543]; properly firm, that is, (figuratively) trustworthy; adverbially surely (often as interjection so be it): - amen, verily.
These two sites give similar information, with the definition from several dictionaries and statistics on the use of the word.
Perseus Digital Library
University of Chicago's Logion lexicon
Thayer's dictionary plus other information.
From this site's dictionary (in Italian)
Click on the first column to search for that word as a form of the root ἀμήν; click on the second column to search for that grammatical form of the root ἀμήν; click on the third column to search for that word and grammatical form; click on the total to search for the root ἀμήν.