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, seen as 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.

The Algerian Constitution of 2016 contains clauses about the term limit and the duration of the presidential term.

Article 226 of the Constitution of Egypt, defining the amendment procedure, ends with an entrenched clause stating that "In all cases, texts pertaining to the re-election of the president of the republic or the principles of freedom and equality stipulated in this Constitution may not be amended, unless the amendment brings more guarantees."

This clause failed to block the 2019 amendments that replaced the hard two-term limit for presidents with a consecutive one, and the modification of the term's duration from four to six years. The article also failed to block a new article from being added that excludes incumbent president Abdel Fattah el-Sisi from the two consecutive terms constraint, enabling him to run for a third term.

In the Constitution of Morocco, eternity clauses exist that ensure certain provisions cannot be amended, including the role of Islam in the nation's law, and the role of the King of Morocco in law.

There are several examples of entrenched clauses that ultimately failed in their objectives, since their protections were undermined in unintended ways. For instance, the South Africa Act, the initial constitution of the Union of South Africa, contained entrenchment clauses protecting voting rights in the Cape Province, including those of some Coloureds, that required two-thirds of a joint session of parliament to be repealed. The Coloureds, however, later lost their voting rights after the Government restructured the Senate and packed it with its sympathisers so that they were able to achieve said supermajority in what is known as the Coloured vote constitutional crisis.