Side EffectA term: an operation is said to have side effects if it modifies the context in which it is executed. Otherwise, it is said to be pure, or reversible. This is often tied to the notion that a function has an input and output signature which delineates its effects from its side-effects.
This notion depends on the space of variables considered as meaningful: a pure function in some context may not be so pure when you consider its implementation.
For example, if you consider that storage is limited, then no function is pure, except one that does strictly nothing, because all functions must consume memory to store their result.