When referring specifically to the United States Constitution, it is the right granted by Article V of the Constitution to propose and ratify amendments to the Constitution.

Of this writing, there are twenty-seven amendments, the last enacted in 1992 (it prohibited Congress from enacting pay raises for itself which would take effect in the selfsame term).

The text of Article V is, as follows (with the important parts highlighted):

Article V

The Congress, whenever two thirds of both Houses shall deem it necessary, shall propose Amendments to this Constitution, or, on the Application of the Legislatures of two thirds of the several States, shall call a Convention for proposing Amendments, which, in either Case, shall be valid to all Intents and Purposes, as Part of this Constitution, when ratified by the Legislatures of three fourths of the several States, or by Conventions in three fourths thereof, as the one or the other Mode of Ratification may be proposed by the Congress; Provided that no Amendment which may be made prior to the Year One thousand eight hundred and eight shall in any Manner affect the first and fourth Clauses in the Ninth Section of the first Article; and that no State, without its Consent, shall be deprived of its equal Suffrage in the Senate.

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