Avoid Effect: An effect that stops another effect from resolving.
Any ability of any type can also be a prevent or avoid ability. Prevent and avoid abilities can stop another effect from happening, and they are the only kinds of abilities that can interrupt another ability during resolution— even when the player who owns the prevent/avoid ability does not have priority— because they can only be triggered when the effect that would be prevented/avoided is resolving.:Ruling:Ruling
Some card abilities use the words "prevent" or "avoid." Prevent or avoid effects are the only effects which can disrupt another effect. A prevent or avoid effect states what it is preventing or avoiding, and an effect that is prevented or avoided is not resolved. Prevent or avoid effects can be triggered whenever the effect they are preventing or avoiding is resolving.
So a cost can be paid even if the effect does nothing, right? You cannot use prevent or avoid effects unless the condition that they are preventing/avoiding is occurring. You could use a non-prevent/avoid paid ability such as on Corroder to spend 1 credit when the Corp rezzes Tollbooth. We will take another look at this scenario before releasing the next FAQ, but that is currently a valid interaction.