A type of custom that cannot be avoided.
E.g. a persistent objector cannot opt out of a jus cogens norm.
Jus cogens is in contravention on a strict positivist approach because it does not seem to based on state consent because states are bound by these norms even if they do not consent to them. The existence of jus cogens still remains somewhat controversial, however they have achieved quite an wide scope of acceptance. The notion of jus cogens is that these norms are owed by states to all of the international law community.
Vienna Convention (VCLT) is an important document that references jus cogens norms. They have also been supported and referenced in ICJ decisions. They are regarded to obligations erga omens i.e. owed to all. There is some controversy as to which rules of customary law are jus cogens norms. There is a short list of generally accepted jus cogens norms: you cant invade another state unless you have security council approval or are acting in self defence (prohibition on use of force); the international crimes under IL such as slavery, piracy, apartheid, torture.