1) an earnest
1a) money which in purchases is given as a pledge or downpayment that the full amount will subsequently be paid
Part of Speech: noun masculine
Citing in TDNT: 1:475, 80
Of Hebrew origin [H6162]; a pledge, that is, part of the purchase money or property given in advance as security for the rest: - earnest.
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 ἀρραβών; click on the second column to search for that grammatical form of the root ἀρραβών; click on the third column to search for that word and grammatical form; click on the total to search for the root ἀρραβών.