What is Special Master?

Legal Definition
In the law of the United States, a special master is generally a subordinate official appointed by a judge to make sure that judicial orders are actually followed, or in the alternative, to hear evidence on behalf of the judge and make recommendations to the judge as to the disposition of a matter. The special master should not be confused with the traditional common law concept of a master, a judge of the High Court entrusted to deal with summary and administrative matters falling short of a full trial.

In the federal judiciary of the United States, a special master is an adjunct to a federal court. Rule 53 of the Federal Rules of Civil Procedure allows a federal court to appoint a master, with the consent of the parties, to conduct proceedings and report to the Court.
-- Wikipedia
Legal Definition
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 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.
Legal Definition
A master in chancery appointed to act as the representative of the court in some particular act or transaction, as, to make a sale of property under a decree. Guaranty Trust, etc., Co. v. Delta & Pine Land Co.., 104 Fed. 5, 43 C. C. A. 396; Pewabic Min. Co. v. Mason. 145 U. S. 349, 12 Sup. Ct. 887, 36 L. Ed. 732.
-- Black's Law Dictionary