Rendition and extraordinary rendition are not terms defined by law. They describe the process by which a detainee is transferred from one state to another, outside normal legal processes (i.e. extradition, deportation, etc). In many cases these detainees are transferred to secret detention or to a third country for the purposes of interrogation, often in circumstances where they face a real risk of torture.
On 5 December 2005 US Secretary of State Condoleezza Rice confirmed that the United States was using renditions to transfer suspected terrorists "from the country where they were captured to their home country or to other countries where they can be questioned, held, or brought to justice". You can read the full statement here.
On 6 September 2006 US President George Bush set out more details of the US rendition programme. He confirmed that, in a separate programme run by the CIA, some suspected terrorists were held in secret locations outside the United States and interrogated using an "alternative set of procedures". You can read this statement in full here.
Prohibition on Torture
There is an absolute prohibition on torture in international and domestic law.
The UK has signed and ratified the UN Convention against Torture and other Cruel Inhuman Degrading Treatment or Punishment. The Convention entered into force on 26 June 1987.
Article 3 of the European Convention on Human Rights prohibits torture. The Human Rights Act 1988, which came into force in 2000, incorporated the ECHR into the UK's domestic law.
Torture is a criminal offence under section 134 of the Criminal Justice Act 1988.
In A and Others v Secretary of State for the Home Department (No. 2) UKHL 71, 2006, the House of Lords ruled that evidence obtained by torture is inadmissible in any legal proceedings in the UK.