digit sum

noun2

1. •
   The sum of all digits.

   • The digit sum of 517 is 5 + 1 + 7 = 13.
2. •
   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.