A "special master" is appointed by a court to carry out some sort of action on its behalf. Theoretically, a "special master" is distinguished from a "master". A master's function is essentially investigative, compiling evidence or documents to inform some future action by the court, whereas a special master carries out some direct action on the part of the court. It appears, however, that the "special master" designation is often used for people doing purely investigative work, and that the simple "master" designation is falling out of use.
Activities carried out by special masters are as diverse as the actions taken by courts. They are often appointed as facilitators in child custody cases, for example, but the term "special master" was also used to describe the person appointed by Congress to administer compensation for the victims of the 9/11 terrorist attacks in New York. The term often appears in original jurisdiction cases decided by the Supreme Court; these are often cases involving boundary disputes between the states, with a special master appointed to resolve questions of geography or historical claims. See, for example, New Jersey v. New York, 523 US 767 (1998)
In U.S. v. Microsoft, Judge Jackson appointed Lawrence Lessig as a special master (as an amicus curiae) to advise the court about technical issues, and to investigate certain claims, such as Microsoft's assertion that removing Internet Explorer from the Windows operating system would make the system slower.
Infrequently, attorneys taking a deposition in a distant, non-courthouse, location may anticipate that a witness will refuse to testify, or that some other problem will come up. For good cause shown, judges may appoint a special master to appear at the deposition to make evidentiary rulings on the spot.