Source : Free On-Line Dictionary of Computing

side-effect
     
        A language construct that modifies the state of the system.
        The most common side-effects are {assignment}, input and
        output.  A language without side-effects is
        {purely-functional} - execution consists of the evaluation of
        an expression and all subexpressions are {referentially
        transparent}.
     
        (1995-02-16)