The definition of a grand jury is a large group of people who investigate a crime and decide if there is evidence for a trial.
An example of a grand jury is the group, lead by Ken Starr, that investigated president Bill Clinton.
a special jury of a statutory number of citizens, usually more than 12, that investigates accusations against persons charged with crime and indicts them for trial before a petit jury if there is sufficient evidence
A jury convened to determine if there is sufficient evidence to warrant the indictment of a suspected offender. A federal grand jury consists of between 16 and 23 persons and is required by the Fifth Amendment to the US Constitution before a person can be indicted for a serious criminal offense under federal law.