An entrenched clause or entrenchment clause of a constitution is a provision that makes certain amendments either more difficult or impossible to pass. Overriding an entrenched clause may require a supermajority, a referendum, or the consent of the minority party. The term eternity clause is used in a similar manner in the constitutions of Brazil, the Czech Republic, Germany, Greece, India, Iran, Italy, Morocco, Norway, and Turkey, but specifically applies to an entrenched clause that can never be overridden. However, if a constitution provides for a mechanism of its own abolition or replacement, like the German Basic Law does in Article 146, this by necessity provides a "back door" for getting rid of the "eternity clause", too.

Any amendment to a constitution that would not satisfy the prerequisites enshrined in a valid entrenched clause would lead to so-called "unconstitutional constitutional law"—that is, an amendment to constitutional law text that appears constitutional by its form, albeit unconstitutional due to the procedure used to enact it or due to the content of its provisions.

Entrenched clauses are, in some cases, justified as protecting the rights of a minority from the dangers of majoritarianism. In other cases, the objective may be to prevent amendments to the constitution that would pervert the fundamental principles it enshrines. However, entrenched clauses are often challenged by their opponents as being undemocratic.