What is Material Witness?

Legal Definition
A material witness (in American law) is a person with information alleged to be material concerning a criminal proceeding. The authority to detain material witnesses dates to the First Judiciary Act of 1789, but the Bail Reform Act of 1984 most recently amended the text of the statute, and it is now codified at 18 U.S.C. ยง 3144. The most recent version allows material witnesses to be held to ensure the giving of their testimony in criminal proceedings or to a grand jury.

Since September 11, 2001, the U.S. has used the material witness statute to detain suspects without charge for indefinite periods of time, often under the rubric of securing grand-jury testimony. This use of the statute is controversial and is currently under judicial review. In Ashcroft v. al-Kidd (2011), the detainee was never charged or called as a witness, and sued then-U.S. Attorney General John Ashcroft. The U.S. Supreme Court overturned a ruling by the Ninth Circuit Court of Appeals and held that Ashcroft qualified for immunity because of his official position.
-- Wikipedia
Legal Definition
the name that is given to the witness who can give information no one else can and is important to the case of the prosecution.