1) a change of mind, as it appears to one who repents, of a purpose he has formed or of something he has done
Part of Speech: noun feminine
Citing in TDNT: 4:975, 636
From G3340; (subjectively) compunction (for guilt, including reformation); by implication reversal (of [another's] decision): - repentance.
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 μετάνοια; click on the second column to search for that grammatical form of the root μετάνοια; click on the third column to search for that word and grammatical form; click on the total to search for the root μετάνοια.