What fraction of both houses must vote to override a veto?

Two thirds of both houses must vote to override a veto.

Explanation:

The word veto comes from Latin and literally means 'forbid'. It is used to denote that a certain party has the right to unilaterally stop a certain piece of legislation. A veto, therefore, provides unlimited power to stop changes, but not to adopt them.

In the United States, the president has the ability to veto legislation that has passed through Congress, but this right is not absolute. A qualified majority of 2/3 of both houses can pass a law, even against a presidential veto; However, if the proposed law has only a simple majority, the president's veto is decisive.