side effect

Short definition from Wiktionary

Noun

1. 1
   An unintended consequence, or secondary result, of any action, in addition to the main or intended consequence of that action.
2. 2
   An unintended consequence, or secondary result, of any action, in addition to the main or intended consequence of that action: An unintended consequence of a drug or therapy; usually an adverse effect, not a beneficial effect.
3. 3
   An unintended consequence, or secondary result, of any action, in addition to the main or intended consequence of that action: A change in state outside of the local environment of the function being called.
4. 4
   Alternative spelling of side effect.

Verb

1. 1
   To alter as a side effect.