Read-Only Operator (+) - SQL
The term read-only operator to the mathematical term function. Here I just need to add that the SQL standard reserves the term function for read-only operators that are invoked using prefix notation: an operator name followed by a commalist of argument expressions enclosed in parentheses, as in for example SQRT(2), denoting the positive square root of 2, or SUBSTRING(X FROM 1 FOR 2) denoting the first two characters of the character string denoted by X. The term operator is used for those invoked using infix notation, like + for example.