The Collaborative International Dictionary
Summum bonum \Sum"mum bo"num\ [L.] (Philos.) The supreme or highest good, -- referring to the object of human life.
Wiktionary
n. (context philosophy English) The greatest good; the ultimate importance, the singular end which human beings ought to pursue.
WordNet
n. the supreme good in which all moral values are included or from which they are derived
Wikipedia
Summum bonum is a Latin expression meaning "the highest good", which was introduced by Cicero, to correspond to the Idea of the Good in ancient Greek philosophy. The summum bonum is generally thought of as being an end in itself, and at the same time containing all other goods. The term was used in medieval philosophy and in Kantianism, to describe the ultimate importance, the singular and overriding end which human beings ought to pursue. 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.
Usage examples of "summum bonum".
Thus shalt thou ever be mindful of the Great Work which thou hast undertaken to perform, and thus shalt thou be strengthened to pursue it unto the attainment of the Stone of the Wise, the Summum Bonum, True Wisdom and Perfect Happiness.
So the mind must abandon the dregs of earth and press on towards to God, on whom alone should be fixed the desire of a spiritual man, for whom the least separation from that summum bonum is to be considered a living death and dreadful loss.
Don Quixote then asked him how many children he had, and observed that one of the things wherein the ancient philosophers, who were without the true knowledge of God, placed the summum bonum was in the gifts of nature, in those of fortune, in having many friends, and many and good children.