1) to cite a witness, bring forward a witness, call to witness
1a) to affirm be appeal to God, to declare solemnly, protest
2) to conjure, beseech as in God's name, exhort solemnly
Part of Speech: verb
Citing in TDNT: 4:510, 564
Middle voice from G3144; to be adduced as a witness, that is, (figuratively) to obtest (in affirmation or exhortation): - take to record, testify.
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 μαρτύρομαι; click on the second column to search for that grammatical form of the root μαρτύρομαι; click on the third column to search for that word and grammatical form; click on the total to search for the root μαρτύρομαι.