The Collaborative International Dictionary of English v.0.48:
Summum bonum \Sum"mum bo"num\ [L.] (Philos.) The supreme or highest good, -- referring to the object of human life. [Webster 1913 Suppl.]WordNet (r) 3.0 (2006):
summum bonum n 1: the supreme good in which all moral values are included or from which they are derived