A side-effect of a procedure is something the procedure does that is not its main purpose and that is visible (or whose results are perceivable) from outside the procedure. Side-effects are the sources of many subtle errors in software, that is, the ones that are the most latent and the ones that are most difficult to discover once their symptoms manifest themselves.
Reference:
Ledgard, H., Programming Proverbs, Rochelle Park, NJ: Hayden Book Company, 1975.
