Utilitarianism is a movement in Ethics and Political Philosophy in 19th Century England, which proposed "the greatest good for the greatest number" as the overriding rule in all moral decision.
Utilitarianism is the idea that the moral worth of an action is solely determined by its contribution to overall utility in maximizing happiness or pleasure as summed among all people, i.e. the greatest happiness for the greatest number of people. It starts from the basis that pleasure and happiness are intrinsically valuable, that pain and suffering are intrinsically disvaluable, and that anything else has value only in its causing happiness or preventing suffering. See the section on doctrine of Utilitarianism for more details.
This focus on happiness or pleasure as the ultimate end of moral decisions, makes Utilitarianism a type of Hedonism (and it is sometimes known as Hedonistic Utilitarianism), and its origins are often traced back to the Epicureanism of the followers of the Greek philosopher Epicurus. It can be argued that David Hume and Edmund Burke were proto-Utilitarians.
As a specific school of thought, however, Utilitarianism is generally credited to the English philosopher and social reformer Jeremy Bentham. Bentham found pain and pleasure to be the only intrinsic values in the world, and from this he derived the rule of utility: that the good is whatever brings the greatest happiness to the greatest number of people. Bentham himself, however, attributed the origins of the theory to Joseph Priestley (1733 - 1804), the English scientist, theologian and founder of Unitarianism in England.
Bentham's foremost proponents were James Mill (1773 - 1836) and his son John Stuart Mill, who was educated from a young age according to Bentham's principles. In his famous 1861 short work, "Utilitarianism", Mill both named the movement and refined Bentham's original principles. He argued that cultural, intellectual and spiritual pleasures are of greater value than mere physical pleasure as valued by a competent judge (which, according to Mill, is anyone who has experienced both the lower pleasures and the higher).
In his essay "On Liberty" and other works, Mill argued that Utilitarianism requires that any political arrangements satisfy the liberty principle (or harm principle), according to which the only purpose for which power can be rightfully exercised over any member of a civilized community against his will, is to prevent harm to others, a cornerstone of the principles of Liberalism and Libertarianism. Some Marxist philosophers have also used these principles as arguments for Socialism.
Other notable Utilitarians, after Bentham and Mill, include Henry Sidgwick (1838 - 1900), G. E. Moore, Bertrand Russell, Richard Hare (1919 - 2002), J. J. C. Smart (1920 - 2012) and Peter Singer (1946 - ).