A special advocate is a lawyer, usually a barrister or advocate, sometimes a solicitor, who is appointed to represent the interests of a party in closed proceedings, i.e. proceedings from which that party has been excluded.
Special advocates are generally prohibited from discussing any evidence that has been the subject of closed proceedings with the excluded party. They are most often used in the context of cases involving national security but have also been used in other matters, including parole board hearings and data protection claims.