Latin meaning the law of nations. It was a doctrine of custom of international law within the ancient Roman legal system and Western law traditions. It is meant to describe that law which is commonly understood by all “gentes” (peoples or nations) in “reasoned compliance with standards of international conduct.” David J. Bederman, International Law in Antiquity (Cambridge University Press, 2004), p. 85.
It is that law which natural reason has established among all men is equally observed among all nations, and is called the “law of nations,” as being the law which all nations use. See also Ius Gentum – a variation of the same term.