Mill defines utilitarianism as a theory based on the principle that "actions are right in proportion as they tend to promote happiness, wrong as they tend to produce the reverse of happiness."

utilitarianism, in normative ethics, a tradition stemming from the late 18th- and 19th-century English philosophers and economists Jeremy Bentham and John Stuart Mill according to which an action (or type of action) is right if it tends to promote happiness or pleasure and wrong if it tends to produce unhappiness or pain—not just for the performer of the action but also for everyone else affected by it.

Mill defines “utilitarianism” as the creed that considers a particular “theory of life” as the “foundation of morals” (CW 10, 210). His view of theory of life was monistic: There is one thing, and one thing only, that is intrinsically desirable, namely pleasure.