The UN Security Council has 15 members, but five have a special status: China, France, Russia, the United Kingdom and the United States are permanent members.
The “veto” comes from Article 27 of the UN Charter and the practice built around it. For substantive decisions, the Council needs at least nine affirmative votes and no negative vote from a permanent member. If one permanent member votes no on such a draft, it fails even if it has the support of the other members.
Procedural matters work differently. They also require nine affirmative votes, but a permanent member cannot block a purely procedural decision with a veto.
An abstention by a permanent member is not the same as a veto. A draft can still pass if it receives the required nine affirmative votes and a permanent member abstains rather than voting no.
Why were five states given this privilege? The Security Council says the UN Charter’s drafters gave special permanent status to China, France, the Soviet Union, the United Kingdom and the United States because of their central roles in creating the post-Second World War United Nations and in maintaining international peace and security. Russia later succeeded the Soviet Union’s seat.
The system was designed around the idea that the UN could not effectively enforce major peace and security decisions against the united opposition of one of the dominant powers. Critics argue that the veto can paralyse the Council and create unequal influence. Supporters of the original design argue that major-power consent was the political price of keeping those powers inside a collective security system.
The veto does not apply to the General Assembly, and permanent members do not have a special veto there.
So when a Security Council resolution “fails because of a veto”, the important questions are whether the matter was substantive, whether at least nine members voted in favour, and whether a permanent member cast a negative vote.