Lessening or restriction of the authority, strength, or power of a law, right, or obligation. Specifically:
Following the terrorist attacks on the USA of 11 September 2001 and the passing of the Antiterrorism, Crime and Security Act 2001, the UK government derogated from Article 5 of the Convention. This derogation was necessary because that Act allowed the detention by the Secretary of State of certain foreign nationals on the basis that they were suspected of involvement in terrorism. That provision was declared to be in breach of the Convention by the House of Lords in December 2004 (A v Secretary of State for the Home Department  UKHL 56,  2 AC 68) and abandoned in March 2005.