In the Catholic Church, this is a letter of recommendation by a priest's own ordinary (Incardination) or superior for admission by the church rector of a different church to celebrate the Eucharist there. A celebret may not be more than a year old (cf. c. 903 CIC/1983; c. 703 CCEO) and is intended to prevent unsanctioned celebrations (cf. c. 1378 § 2, 1° CIC/1983; c. 1443 CCEO).
K. Lüdicke, MKCIC, canon 903 (as of Nov 1989)
E. Miragoli, “Il celebret,” Quaderni di diritto ecclesiale 7, 1994, 435–442.