A reservation in international law is a caveat to a state's acceptance of a treaty. A reservation is defined by the 1969 Vienna Convention on the Law of Treaties (VCLT) as:
a unilateral statement, however phrased or named, made by a State, when signing, ratifying, accepting, approving or acceding to a treaty, 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))[1]
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 cannot take reservations after they have accepted the treaty; a reservation must be made at the time that the treaty affects the State. The Vienna Convention did not create the concept of reservations but codified existing customary law. Thus even States that have not formally acceded to the Vienna Convention act as if they had. As reservations are defined under the Vienna Convention and interpretative declarations are not, the two are sometimes difficult to discern from each other. Unlike a reservation, a declaration is not meant to affect the State's legal obligations but is attached to State's consent to a treaty to explain or interpret what the State deems unclear.