[Haskell-beginners] explaining effects

[martin]

Am 12/15/2015 um 09:55 AM schrieb Alexander Berntsen:
> FWIW, I say "effect" rather than "side effect" when talking about Haskell, because in Haskell effects happen when
> you want them, not as an unforeseen side-effect as a result of the complexity inherent to the source code.
> 
> It is often said that having an effect is "difficult" in Haskell. But really, it's just that if you are launching
> missiles in Haskell, *you actually mean to*. It didn't happen because you wanted to increment i and then "oops,
> stuff happened".

What is the exact defintion of "effect". Everybody talks about it but I am certainly unable to give a defintion.