An function or expression is said to have a side effect if it modifies some state variable values outside its local environment, that is to say has an observable effect besides returning a value (which is the main effect) to the invoker of the operation. State data updated "outside" of the operation may be maintained "inside" a stateful object or a wider stateful system within which the operation is performed.
Example side effects include modifying a non-local variable, modifying a static local variable, modifying a mutable argument passed by reference, performing I/O or calling other side-effect functions.
In the presence of side effects, a program's behaviour may depend on history; that is, the order of evaluation matters. Understanding and debugging a function with side effects requires knowledge about the context and its possible histories.