Jus Cogens means "Compelling Law " in Latin
It is a "higher law" that may not be violated by any country. For example, genocide or slave trade goes against jus cogens.
Under Article 53 of the Vienna Convention, a jus cogens norm is:
"a norm accepted and recognized by the international community of States as a whole as a norm from which no derogation is permitted and which can be modified only by a subsequent norm of general international law having the same character."
" The Restatement (Third) of the Foreign Relations Law agrees with this standard, asserting that the norm is established where there is acceptance and recognition by a "large majority" of states, even if over dissent by "a very small number of states" "
An extract from Summary of Customary International Law and Jus Cogens as Pertains to Juvenile Offenders follows :
http://www.internationaljusticeproject.org/juvJusCogens.cfm
( BTW, the whole article is worth reading, especially if you are interested in criminology.
"The idea of international jus cogens as a body of 'higher law' of overriding importance for the international community is steadily gaining ground.
First embodied in the 1969 Vienna Convention on the Law of Treaties, it was recently confirmed by the 1986 Vienna Convention on the Law of Treaties.
In its judgment in the Nicaragua Case the International Court of Justice (ICJ) clearly affirmed jus cogens as an accepted doctrine in international law.
The growing acceptance of the jus cogens doctrine is also reflected in the increased reliance on specific peremptory rules in the official argumentation of governments. From a law-making perspective, of major importance is the fact that States developed a tendency to rely on the concept of jus cogens in their efforts to achieve profound changes in the existing law. States pressing for the rapid reforms in the existing international legal order regard the concept as a powerful tool of renovation. "
And some interesting jurisprudential considerations:
" ...Naturalists always taught that states cannot be absolutely free in establishing their contractual relations. They were obliged to respect certain fundamental principles deeply rooted in the international community. It is not surprising, therefore, that the negotiations on jus cogens were accompanied by assertions of the continued importance of natural law. At the 1969 Vienna Conference on the Law of Treaties a number of states stressed the fact that jus cogens derived its origin from concepts of natural law.
Many participants of the negotiations believed that rules of jus cogens are based on the legal conscience and moral beliefs of mankind. The acceptance of the jus cogens doctrine was perceived as a major crisis of legal positivism."
