A mandatory legal standard from which no derogation, in domestic law or international law, is allowed.
As the United States District Court wrote in Smith:
"Jus cogens ... is an international law principle which is accepted by the international community of states as a whole as a norm from which no derogation is permitted."
A concept, principally of international law, which distinguishes between law from which domestic deviations may be permitted (aka jus disposititum) and fundamental, universally accepted standards of international law such as prohibitions against slavery, torture or genocide.
In the Vienna Treaty on the Law of Treaties 1969, at §53, jus cogens is defined as a:
"... peremptory norm of general international law.
"For the purposes of the present Convention, a peremptory norm of general international law 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.
"A treaty is void if, at the time of its conclusion, it conflicts with a peremptory norm of general international law."
In Siderman de Blake, Justice Fletcher of the U.S. 9th Circuit Court of Appeals adopted these words:
"While jus cogens and customary international law are related, they differ in one important respect.
"Customary international law, like international law defined by treaties and other international agreements, rests on the consent of states. A state that persistently objects to a norm of customary international law that other states accept is not bound by that norm ... just as a state that is not party to an international agreement is not bound by the terms of that agreement. International agreements and customary international law create norms known as jus dispositivum, the category of international law that consists of norms derived from the consent of states and that is founded on the self-interest of the participating states.
"In contrast, jus cogens embraces customary laws considered binding on all nations, and is derived from values taken to be fundamental by the international community, rather than from the fortuitous or self-interested choices of nations. Whereas customary international law derives solely from the consent of states, the fundamental and universal norms constituting jus cogens transcend such consent, as exemplified by the theories underlying the judgments of the Nuremberg tribunals.... The legitimacy of the Nuremberg prosecutions rested not on the consent of the Axis Powers and individual defendants, but on the nature of the acts they committed: acts that the laws of all civilized nations define as criminal. The universal and fundamental rights of human beings identified by Nuremberg - rights against genocide, enslavement, and other inhumane acts - are the direct ancestors of the universal and fundamental norms recognized as jus cogens....
"(T)hese norms, which include principles and rules concerning the basic rights of the human person, are the concern of all states....
"... the right to be free from official torture is fundamental and universal, a right deserving of the highest status under international law, a norm of jus cogens."
The body of jus cogens is growing - evolving, as more and more basic human rights emerge within the domestic law of states and find their way into first drafts then final versions of proposed international treaties. Indeed, at §64 of the Vienna Treaty:
"If a new peremptory norm of general international law emerges, any existing treaty which is in conflict with that norm becomes void and terminates."
But the lack of stability in this area is a significant concern of courts called upon by lawyers to incorporate jus cogens in their rationale decindendi.
Canada's Supreme Court, in Suresh, at §63 , in a case where a prohibition against torture was recognized as jus cogens, and defined as:
"Peremptory norms develop over time and by general consensus of the international community.
"This is the difficulty in interpreting international law; it is often impossible to pinpoint when a norm is generally accepted and to identify who makes up the international community."
In Peremptory Norms (Jus Cogens) in International Law: Historical Development, Criteria, Present Status, Lauri Hannikainen wrote:
"The clarification of the notion of jus cogens in international law is advancing, but is still far from being completed.
"On the other hand, the international community of States has been inactive in stating expressly which norms it recognizes as peremptory in the present-day international law. .... (T)his inactivity, and the consequent uncertainty as to which norms are peremptory, constitute at present the main problem of the viability of jus cogens."
- Duhaime, Lloyd, International Law
- Duhaime, Lloyd, International Law • A Primer
- Duhaime, Lloyd, Legal Definition of Jus Dispositivum
- Hannikainen, L., Peremptory Norms (Jus Cogens) in International Law: Historical Development, Criteria, Present Status (Helsinki: Finnish Lawyers’ Publishing Co., 1988).
- Sarei v. Rio Tinto, PLC., 456 F. 3d 1069 (2006) "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."
- Siderman de Blake v Argentina, 965 F. 2d 699 (U.S. 9th Circuit Court of Appeals, 1992). Retrieved from derechos.org/nizkor/arg/ley/siderman.html on October 31, 2009.
- Smith v Socialist People's Libyan Arab Jamahiriya, 886 F. Supp. 306 (1995)
- Suresh v Canada 2002 SCC 1; also at  1 SCR 3