pure function
In computing, a function that always returns the same output for the same input and has no side effects.
Pure functions are easy to test because they have no side effects.
Side effects are what pure functions avoid; 副作用 is often contrasted with 純粋関数 in programming contexts.
Compound of 純粋 (pure) and 関数 (function), a direct translation of the English computing term 'pure function'.