Pure function

In computer programming, a pure function is a function that has the following properties:

1. the function return values are identical for identical arguments (no variation with local static variables, non-local variables, mutable reference arguments or input streams, i.e., referential transparency), and
2. the function has no side effects (no mutation of local static variables, non-local variables, mutable reference arguments or input/output streams).

Some authors, particularly from the imperative language community, use the term "pure" for all functions that just have the above property 2 (discussed below).