a peremptory norm (also called jus cogens or ius cogens /; latin for "compelling law") is a fundamental principle of international law that is accepted by the international community of states as a norm from which no derogation is permitted.
there is no universal agreement regarding precisely which norms are jus cogens nor how a norm reaches that status, but it is generally accepted that jus cogens bans genocide, maritime piracy, enslaving in general (i.e. slavery as well as slave trade), wars of aggression and territorial aggrandizement, torture and refoulement. the latter two are evolving and controversial as mainly rest on the definition of torture in regards to criminal sentencing. if sentencing is not cruel, inhuman or degrading but arbitrary or disproportionate convictions are imposed then a state's refoulement — where limited to the returning of unsubstantiated asylum claimants — may still be lawfully conducted to many such countries which are juridically developing, such as those lacking a clear separation of powers, with a relatively heightened risk of political persecution and reports of unfair trials.