WordNet (r) 3.0 (2006):

grand jury
    n 1: a jury to inquire into accusations of crime and to evaluate
         the grounds for indictments

Bouvier's Law Dictionary, Revised 6th Ed (1856):

GRAND JURY, practice. A body of men, consisting of not less than twelve nor
more than twenty-four, respectively returned by the sheriff of every county
to every session of the peace, oyer and terminer and general gaol delivery,
to whom indictments are preferred. 4 Bl. Com. 302; 1 Chit. C. L. 310, 1.
     2. There is just reason to believe that this institution existed among
the Saxons, Crabb's C. L. 35. By the constitutions of Clarendon, enacted 10
H. II. A. D. 1164, it is provided, that "if such men were suspected, whom
none wished or dared to accuse, the sheriff, being thereto required by the
bishop, should swear twelve men of the neighborhood, or village, to declare
the truth" respecting such supposed crime; the jurors being summoned as
witnesses or accusers, rather than judges. If this institution did not exist
before, it seems to be pretty certain that this statute established grand
juries, or recognized them, if they existed before.
     3. A view of the important duties of grand juries will be taken, by
considering, 1. The organization of the grand jury. 2. The extent of its
jurisdiction. 3. The mode of doing business. 4. The evidence to be received.
5. Their duty to make presentments. 6. The secrecy to be observed by the
grand jury.
     4. - 1. Of the organization of the grand jury. The law requires that
twenty-four citizens shall be summoned to attend on the grand jury; but in
practice, not more than twenty-three are sworn, because of the inconvenience
which else might arise, of having twelve, who are sufficient to find a true
bill, opposed to twelve others who might be against it. 6 Adolph. & Ell.
236; S. C. 33 e. C. L. R. 66; 2 Caines, R. 98. Upon being called, all who
present themselves are sworn, as it scarcely ever happens that all who are
summoned are in attendance. The grand jury cannot consist of less than
twelve, and from fifteen to twenty are usually sworn. 2 Hale, P. C. 161; 7
Sm. & Marsh. 58. Being called into the jury box, they are usually permitted
to select a foreman whom the court appoints, but the court may exercise the
right to nominate one for them. The foreman then takes the following oath or
affirmation, namely: "You A B, as foreman of this inquest for the body of
the ______ of _________, do swear, (or affirm) that you will diligently
inquire, and true presentments make, of all such articles, matters and
things as shall be given you in charge, or otherwise come to your knowledge
touching the present service; the commonwealth's counsel, your fellows and
your own, you shall keep secret; you shall present no one for envy, hatred
or malice; nor shall you leave any one unpresented for fear, favor,
affection, hope of reward or gain; but shall present all things truly, as
they come to your knowledge, according to the best of your understanding,
(so help you God.") It will be perceived that this oath contains the
substance of the duties of the grand jury. The foreman having been sworn or
affirmed, the other grand jurors are sworn or affirmed according to this
formula: "You 'and each of you do swear (or affirm) that the same oath (or
affirmation) which your foreman has taken on his part, you and every one of
you shall well and truly observe on your part." Being so sworn or affirmed,
and having received the charge of the court, the grand jury are organized,
and may proceed to the room provided for them to transact the business which
may be laid before them. 2 Burr. 1088; Bac. Ab. Juries, A. The grand jury
constitute a regular body until discharged by the court, or by operation of
law, as where they cannot continue by virtue of an act of assembly beyond a
certain day. But although they have been formally discharged by the court,
if they have not separated, they may be called back, and fresh bills
submitted to them; 9 C. & P. 43; S. C. 38 E. C. L. R. 2 8.
     5. - 2. The extent of the grand jury's jurisdiction. Their jurisdiction
is coextensive with that of the court for which they inquire; both as to the
offences triable there, and the territory over which such court has
jurisdiction.
     6. - 3. The mode of doing business. The foreman acts as president, and
the jury usually appoint one of their number to perform the duties of
secretary. No records are to be kept of the acts of the grand jury, except
for their own use, because, as will be seen hereafter, their proceedings are
to be secret. Being thus prepared to enter upon their duties, the grand jury
are supplied with bills of indictment by the attorney-general or other
officer, representing the state or commonwealth against offenders. On these
bills are endorsed the names of the witnesses by whose testimony they are
supported. The witnesses are in attendance in another room, and must be
called when wanted. Before they are examined as to their knowledge of the
matters mentioned in the indictment, care must be taken that they have been
sworn or affirmed. For the sake of convenience, they are generally sworn or
affirmed in open court before they are sent to be examined, and when so
qualified, a mark to that effect is made opposite their names.
     7. In order to save time, the best practice is to find a true bill, as
soon as the jury are satisfied that the defendant ought to be put upon his
trial. It is a waste of time to examine any other witness after they have
arrived at that conclusion. Twelve at least must agree, in order to find a
true bill; but it is not required that they should be unanimous. Unless that
number consent, the bill must be ignored. When a defendant is to be put upon
his trial, the foreman must write on the back of the indictment "a true
bill," sign his name as foreman, and date the time of finding. On the.
contrary, where there is not sufficient evidence to authorize the finding of
the bill, the jury return that they are ignorant whether the person accused
committed the offence charged in the bill, which is expressed by the foreman
endorsing on the bill "ignoramus," signing his name as before, and dating
the time.
     8. - 4. Of the evidence to be received. In order to, ascertain the
facts which the jury have not themselves witnessed, they must depend upon
the statement of those who know them, and who will testify to them. When the
witness, from his position and ability, has been in a condition to know the
facts about which he testifies, he is deserving of implicit confidence; if,
with such knowledge, he has no motive for telling a false or exaggerated
story, has intelligence enough to tell what he knows, and give a probable
account of the transaction. If, on the other hand, from his position he
could not know the facts, or if knowing them, he distorts them, he is
undeserving of credit. The jury are the able judges of the credit and
confidence to which a witness is entitled.
     9. Should any member of the jury be acquainted with any fact on which
the grand jury are to act, he must, before he testifies, be sworn or
affirmed, as any other witness, for the law requires this sanction in all
cases.
    10. As the jury are not competent to try the accused, but merely to
investigate the case so far as to ascertain whether he ought to be put on
his trial, they cannot hear evidence in his favor; theirs is a mere
preliminary inquiry; it is when he comes to be tried in court that he may
defend himself by examining witnesses in his favor, and showing the facts of
the case.
    11. - 5. Of presentments. The jury are required to make true
presentments of all such matters which may be given to them in charge, or
which have otherwise come to their knowledge. A presentment, properly
speaking, is the notice taken by the grand jury of any offence from their
own knowledge, as of a nuisance, a libel, or the like. In these cases, the
authors of the offence should be named, so that they may be indicted,
    12. - 6. Of the secrecy to be observed by the grand jury. The oath which
they have taken obliges them to keep secret the commonwealth's counsel,
their fellows and their own. Although contrary to the general spirit of our
institutions, which do not shun daylight, this secrecy is required by law
for wise purposes. It extends to the votes given in any case, to the
evidence delivered by witnesses, and the communications of the jurors to
each other; the disclosure of these facts, unless under the sanction of law,
would render the imprudent juror who should make them public, liable to
punishment. Giving intelligence to a defendant that a bill has been found
against him, to enable him to escape, is so obviously wrong, that no one can
for a moment doubt its being criminal. The grand juror who should be guilty
of this offence might, upon conviction, be fined and imprisoned. The
duration of the secrecy appears not to be definitely settled, but it seems
this injunction is to remain as long as the particular circumstances of each
case require. In a case, for example, where a witness swears to a fact in
open court, on the trial, directly in opposition to what he swore before the
grand jury, there can be no doubt the injunction of secrecy, as far as
regards this evidence, would be at an end, and the grand juror might be
sworn to testify what this witness swore to in the grand jury's room, in
order that the witness might be prosecuted for perjury. 2 Russ. Cr.. 616; 4
Greenl. Rep. 439; but see contra, 2 Halst. R. 347; 1 Car. & K. 519. Vide,
generally, 1 Chit. Cr. Law, 162; 1 Russ. Cr. 291; 2 Russ. Cr. 616 2 Stark.
Ev. 232, n. 1; 1 Hawk. 65, 500 2 Hawk. ch. 25; .3 Story, Const. Sec. 1778 2
Swift's Dig. 370; 4 Bl. Com. 402; Archb. Cr. Pl. 63; 7 Sm. Laws Penna. 685.