[math-fun] What is the name of [mod 9 digitsum]

Hello Math-Fun, is there a name for this operation : - start with 178 for instance; - add the digits: 1+7+8 => 16 - add the digits again: 1+6 => 7 Does the operation going from 178 to 7 have a name (other than "modulus 9 digitsum")? Thank you, Best, É.