Wiktionary
additive function
n. (context mathematics English) A unary function that preserves the addition operation; a unary function ''f'' such that for any ''x'' and ''y'': ''f''(''x'' + ''y'') = ''f''(''x'') + ''f''(''y'').
Wikipedia
Additive function
In number theory, an additive function is an arithmetic function f(n) of the positive integer n such that whenever a and b are coprime, the function of the product is the sum of the functions:
f(ab) = f(a) + f(b).