1a) of the business of money changers, exchanging equivalent values
2) adjustment of a difference, reconciliation, restoration to favour
2a) in the NT of the restoration of the favour of God to sinners that repent and put their trust in the expiatory death of Christ
Part of Speech: noun feminine
Citing in TDNT: 1:258, 40
From G2644; exchange (figuratively adjustment), that is, restoration to (the divine) favor: - atonement, reconciliation (-ing).
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 καταλλαγή; click on the second column to search for that grammatical form of the root καταλλαγή; click on the third column to search for that word and grammatical form; click on the total to search for the root καταλλαγή.