Wiktionary
n. (context legal English) A person with information alleged to be material concerning a criminal proceeding, and who therefore may be detained to insure their participation in such a proceeding.
WordNet
n. a witness whose testimony is both relevant to the matter at issue and required in order to resolve the matter
Wikipedia
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 . 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.
Usage examples of "material witness".
Cramer has been itching for years to lock you up, and any judge would commit you as a material witness who had run out.
And the negative judgment on the police would be that they blew it: they never should have let a potential victim and/or material witness out of their protection.
He knows you were called in as part of the Nite Owl reopening, so he knows he's a material witness at least.
Well, I could summon him back from the beach as a material witness-his plane isn't gone yet-and keep him here till hell froze over.
Otherwise, you become a material witness, one who may have compromised the crime scene.
The Tleilaxu pilot who had survived the Atreides attack inside the Heighliner was a material witness at the trial, and thus had been forced to remain on Kaitain.
I sent you to guard that material witness for the prosecutor, not solve his case for him, and who told you to run off with the files on that more serious payroll robbery?