Summum bonum is a Latin expression meaning the highest or ultimate good, which was introduced by the Roman philosopher Cicero to denote the fundamental principle on which some system of ethics is based — that is, the aim of actions, which, if consistently pursued, will lead to the best possible life. Since Cicero, the expression has acquired a secondary meaning as the essence or ultimate metaphysical principle of Goodness itself, or what Plato called the Form of the Good. These two meanings do not necessarily coincide. For example, Epicurean and Cyrenaic philosophers claimed that the 'good life' consistently aimed for pleasure, without suggesting that pleasure constituted the meaning or essence of Goodness outside the ethical sphere. In De finibus, Cicero explains and compares the ethical systems of several schools of Greek philosophy, including Stoicism, Epicureanism, Aristotelianism and Platonism, based on how each defines the ethical summum bonum differently.
The term was used in medieval philosophy. In the Thomist synthesis of Aristotelianism and Christianity, the highest good is usually defined as the life of the righteous and/or the life led in communion with God and according to God's precepts. In Kantianism, it was used to describe the ultimate importance, the singular and overriding end which human beings ought to pursue.