Noun
(mathematics, of a positive integer) The sum of all digits.
The digit sum of 517 is 5 + 1 + 7 = 13.
(mathematics, of a positive integer) the number obtained when repeatedly applying above described operation until arriving at a single-digit number.
The digit sum of 517 is 4, since 5 + 1 + 7 = 13 and 1 + 3 = 4.