1) the chief or main point, the principal thing
2) the pecuniary sum total of a reckoning, amount
2a) the principal, capital, as distinguished from the interest
2b) a sum of money, sum
Part of Speech: noun neuter
Neuter of a derivative of G2776; a principal thing, that is, main point; specifically an amount (of money): - sum.
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 κεφάλαιον; click on the second column to search for that grammatical form of the root κεφάλαιον; click on the third column to search for that word and grammatical form; click on the total to search for the root κεφάλαιον.