Let's say that we have a language that allows us to do the following

x = 4 + 5;             // the integer x now = 9

y = 6.43 + 3.2;        // the real number y now = 9.63

z  = "abc" + "123";    // the string z now = "abc123"

So here we have the + operator being used in a very consistent way, for each of the three types of data. We generally we refer to this element of language design as...?