summum bonum
Concise Oxford English Dictionary © 2008 Oxford University Press:
summum bonum /ˌsʊməm ˈbɒnəm, ˌsʌməm ˈbəʊnəm/
▶noun the highest good, especially as the ultimate goal according to which values and priorities are established in an ethical system.
– origin from L.
'summum bonum' also found in these Oxford entries:

