Utilitarianism is an ethical theory holding that the proper course of action is the one that maximizes the overall "happiness", by whatever means necessary. It is thus a form of consequentialism

Consequentialism

Consequentialism is the class of normative ethical theories holding that the consequences of one's conduct are the ultimate basis for any judgment about the rightness of that conduct...

, meaning that the moral worth of an action is determined only by its resulting outcome, and that one can only weigh the morality of an action after knowing all its consequences. The most influential contributors to this theory are considered to be Jeremy Bentham

Jeremy Bentham

Jeremy Bentham was an English jurist, philosopher, and legal and social reformer. He became a leading theorist in Anglo-American philosophy of law, and a political radical whose ideas influenced the development of welfarism...

 and John Stuart Mill

John Stuart Mill

John Stuart Mill was a British philosopher, economist and civil servant. An influential contributor to social theory, political theory, and political economy, his conception of liberty justified the freedom of the individual in opposition to unlimited state control. He was a proponent of...

.

Utilitarianism was described by Bentham as "the greatest happiness principle".

Utilitarianism can be characterised as a quantitative and reductionist approach to ethics.