Q. 1. What is the meaning of a "celebret?"
A. 1. A celebret is a sort of clerical ID card that a bishop gives to a priest, that he may obtain permission in another diocese to say Mass, and for this purpose bears testimony that he is free from canonical censures. The Council of Trent (Sess. XXIII, chap. xvi on Reform) lays down the rule that "no cleric who is a stranger shall without letters commendatory from his own ordinary be admitted by any bishop to celebrate the Divine Mysteries". Ordinarily permission is not to be given to a priest from another diocese to say Mass without this certificate signed and duly sealed. The seal is obviously the more important requisite, as it is the safer guarantee against forgery. The celebret should be officially recognized by the diocesan authority of the place where a priest may wish to say Mass. One who has his celebret in due form, or who is certainly known to be in good standing in his own diocese, may be allowed to celebrate till he has had a sufficient time to comply with this rule. A priest with proper credentials cannot reasonably be prevented from saying Mass, though he will be expected to comply with reasonable restrictions which may be imposed.
[Source: The Catholic Encyclopedia, Vol. 3. New York: Robert Appleton Company, 1908. 21 Feb. 2016.]
Celebret comes from the Latin, meaning, "Let him celebrate."