side-effect

from The Free On-line Dictionary of Computing (8 July 2008)

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)