1) to speak or tell the truth
1a) to teach the truth
1b) to profess the truth
Part of Speech: verb
Citing in TDNT: 1:251, 37
From G227; to be true (in doctrine and profession): - speak (tell) the truth.
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 ἀληθεύω; click on the second column to search for that grammatical form of the root ἀληθεύω; click on the third column to search for that word and grammatical form; click on the total to search for the root ἀληθεύω.