Utilitarianism refers to a moral theory that states that what is moral right is whatever produces the greatest overall amount of pleasure (hedonistic utilitarianism) or happiness (eudaimonistic utilitarianism).
Some utilitarians (act utilitarians) claim that we should weigh the consequences of each individual action, while others (rule utilitarians) maintain that we should look at the consequences of adopting particular rules of conduct.
Other definition:
Utilitarianism refers to the theory of Jeremy Bentham and John Stuart Mill that the overall utility or benefit produced by an action ought to be the standard by which we judge the worth or goodness of moral and legal action.