Real Dictionary

grand jury
countable noun: A grand jury is a jury, usually in the United States, which considers a criminal case in order to decide if someone should be tried in a court of law.

grand jury in American English
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
noun: a jury, at common law, of 12 to 23 persons, designated to inquire into alleged violations of the law in order to ascertain whether the evidence is sufficient to warrant trial

grand jury in British English
noun: (esp in the US and, now rarely, in Canada) a jury of between 12 and 23 persons summoned to inquire into accusations of crime and ascertain whether the evidence is adequate to found an indictment. Abolished in Britain in 1948