Utilitarianism is a way of deciding what is right and wrong. The rule is simple to say: the best action is the one that brings the most happiness, or the least pain, to the most people. You add up the good and the bad for everyone, and the answer that helps the biggest number wins. Notice what this leaves out — it does not ask if the action feels noble, or if it follows a rule, or if it is what God or tradition says. Only the result counts. In serious use, this is the name of a real school of thought in philosophy, started in England about two hundred years ago by Jeremy Bentham and John Stuart Mill, and it is still argued about today in ethics classes, in law, in medicine (who gets the one machine that can save a life?), and in government policy. Outside philosophy class, people use the word more loosely, and often as a small complaint: if you say a decision was 'pure utilitarianism', you often mean it was cold — that the numbers were counted and the single person who got hurt was not. That is the famous problem with it. If hurting one person makes a hundred people happier, the maths says do it, and most of us feel something is wrong there. So the word carries both the idea and the doubt about the idea. It is a formal, written word. You meet it in books, essays and news articles, not in conversation with friends. Careful: 'utilitarianism' is the belief system; a 'utilitarian' is a person who holds it (or, as an adjective, something plain and useful with no beauty — a utilitarian grey building). Those two uses of 'utilitarian' are related but they are not the same, and the -ism word only means the belief.