A reservation in international law

International law

Public international law concerns the structure and conduct of sovereign states; analogous entities, such as the Holy See; and intergovernmental organizations. To a lesser degree, international law also may affect multinational corporations and individuals, an impact increasingly evolving beyond...

 is a caveat

Caveat

Caveat , the third-person singular present subjunctive of the Latin cavere, means "warning" ; it can be shorthand for Latin phrases such as:...

 to a state's acceptance of a treaty. By the 1969 Vienna Convention on the Law of Treaties

Vienna Convention on the Law of Treaties

The Vienna Convention on the Law of Treaties is a treaty concerning the international law on treaties between states. It was adopted on 22 May 1969 and opened for signature on 23 May 1969. The Convention entered into force on 27 January 1980. The VCLT has been ratified by 111 states as of November...

 (VCLT), a reservation is defined as a

unilateral statement, however phrased or named, made by a State

Sovereign state

A sovereign state, or simply, state, is a state with a defined territory on which it exercises internal and external sovereignty, a permanent population, a government, and the capacity to enter into relations with other sovereign states. It is also normally understood to be a state which is neither...

, when signing, ratifying

Ratification

Ratification is a principal's approval of an act of its agent where the agent lacked authority to legally bind the principal. The term applies to private contract law, international treaties, and constitutionals in federations such as the United States and Canada.- Private law :In contract law, the...

, accepting, approving or acceding to a treaty

Treaty

A treaty is an express agreement under international law entered into by actors in international law, namely sovereign states and international organizations. A treaty may also be known as an agreement, protocol, covenant, convention or exchange of letters, among other terms...

, whereby it purports to exclude or to modify the legal effect of certain provisions of the treaty in their application to that State. (Article 2 (1)(d))

In effect, a reservation allows the state to be a party to the treaty, while excluding the legal effect of that specific provision in the treaty to which it objects.
States can not take reservations after they have accepted the treaty; a reservation must be made at the time that the treaty affects the State.