By Utilitarianism is here meant the ethical theory, that the conduct which, under any given circumstances, is objectively right, is that which will produce the greatest amount of happiness on the whole; that is, taking into account all whose happiness is affected by the conduct.
(Henry Sidgwick, The Methods of Ethics, 7th ed. [Indianapolis: Hackett Publishing Company, 1981], bk. IV, chap. I, sec. 1, p. 411 [first published in 1907; 1st ed. published in 1874])
Note from KBJ: Classical utilitarianism of the sort espoused by Bentham, Mill, and Sidgwick has five features, all of which are mentioned (or suggested) by Sidgwick in this short quotation. First, it is consequentialistic. This means that the rightness of an act is a function solely of its consequences. Those who deny this are called deontologists. Second, it is monistic. This means that there is only one intrinsically good thing. Those who deny this are called pluralists. Third, it is hedonistic. This means that the one intrinsically good thing is happiness (which the classical utilitarians defined, oddly, as pleasure and the absence of pain). Some utilitarians reject hedonism in favor of welfarism, preference satisfaction, or desire satisfaction. Fourth, it is optimific. The right act maximizes the good; it does not merely produce a satisfactory amount of it. Those who deny this are called satisficing utilitarians. Fifth, it is impartialistic. As Sidgwick says, it takes into account "all whose happiness is affected by the conduct" (including nonhuman animals). Those who deny this are called partialists. Egoists are partialists, but so are familialists, clanists, tribalists, racialists, credalists, and nationalists. Think of utilitarianism and egoism as lying at opposite ends of a spectrum, with many intermediate possibilities.