Distinction between ways of applying the greatest happiness principle for the moral evaluation of actions on utilitarian grounds. Act-utilitarianism supposes that each particular action should be evaluated solely by reference to the merit of its own consequences, while rule-utilitarianism considers the consequent value of widespread performance of similar actions.
The act-utilitarian asks, “How much pleasure or pain would result if I did this now?”
The rule-utilitarian asks, “How much pleasure or pain would result if everyone were to do this?”
Since the answers to these questions may be quite different, they may lead to distinct recommendations about moral conduct. Although Mill noted that reliance on moral rules may be of practical use in decision-making, he argued that their influence should remain defeasible in particular circumstances.
(Via Philosophical Dictionary)