Utilitarianism says the right act is whatever maximises total happiness

Jeremy Bentham (1748–1832) proposed that morality could be reduced to a single principle: the 'greatest happiness for the greatest number', measurable in a 'felicific calculus' of pleasure and pain. John Stuart Mill refined it in 1863, distinguishing higher and lower pleasures — poetry beats pushpin, even if pushpin produces more pleasure for more people. The most celebrated objection is Robert Nozick's 'utility monster': an entity that gets vastly more pleasure from consuming resources than others do. Pure utilitarianism would endorse feeding the monster at everyone else's expense — a reductio that has led to hybrid theories ever since.

Source: Bentham, Introduction to the Principles of Morals and Legislation (1789); Mill, Utilitarianism (1863)