An entrenched clause or entrenchment clause of a constitution is a provision which makes certain amendments either more difficult than others or impossible. It may require some form of supermajority, a referendum submitted to the people, or the consent of some other party.
An entrenched clause whose intent is to prevent subsequent amendments, will, once it is adopted, and provided that it is correctly drafted, make some portion of a constitution irrevocable except through the assertion of the right of revolution.
Any amendment to a constitution which would not satisfy the prerequisites enshrined in a valid entrenched clause would lead to so-called "unconstitutional constitutional law", i.e. an amendment to constitutional law text which would appear to be constitutional law only by its form, albeit being unconstitutional as with respect to the procedure in which it has been enacted, or as to the material content of its provisions.
Entrenched clauses are, in some cases, justified as protecting the rights of a minority from the dangers of majoritarianism, but they are often challenged by their opponents as being particularly undemocratic. In other cases, the objective may be to prevent amendments to the constitution which would pervert the fundamental principles enshrined in it, in particular to prevent the creation of a "legalistic" dictatorship.