INDiction. — A chronological computation based on a fifteen-year cycle whose years are progressively numbered from 1 to 15 and indicated respectively as i, II, III, etc. After the 15th year, the numbering begins again from 1, always mentioning only the year of each cycle and disregarding any indication relating to the series of cycles themselves. The indiction is therefore not by itself a sufficient chronological indication, but must be related to other dates (Christian era, years of reign or pontificate, etc.); it does, however, constitute an element of control and is of great importance in diplomatic criticism.
Its use dates back, in Egypt, at least to 297 A.D. (cf. A. Calderini, Papyri, Milan [1944], p. 35 sq.); it seems that the indiction period corresponded, in that region, to the interval between one cadastral revision and another; the computation would have been introduced initially in documents relating to tax impositions and from there extended to other acts: having entered into liturgical practice, its use became general.
The indiction year, like the year of the Christian era, did not everywhere begin on the same day, so that in control checks one must take into account the possibility that the indiction figure may be increased or decreased by one unit depending on the style followed in that time and place. The following styles were in use: a) the Greek, or Constantine-Italian, indiction, beginning on 1 September, which therefore anticipates the year of the Circumcision by four months; it was proper to the court of Byzantium, preferred in papal documents from Pelagius II to Victor III, employed in those of the popes up to 832, widely used in public and private acts of southern Italy; b) the Bedan indiction, so called by Grottefend because first attested in St. Bede, also improperly called Caesarean or Constantinian, beginning on 24 September, which anticipates the year of the Circumcision by three months and six days; it is found in the imperial chancery up to 850, variously in Italy, especially central Italy, and is widely diffused in episcopal documents of Germany up to the middle of the 14th century. In the territory of Genoa it is encountered with a delay of one year, i.e., with one unit less than the Bedan computation as a number; c) the Roman or papal indiction, beginning on 25 December and then on 1 January, coinciding therefore first with the year of the Nativity and then with that of the Circumcision; it is attested in Rome at the end of the 10th century; from 1088 Constantinian in its use in papal letters, in the 13th century it is also found in the German chancery; it then becomes general in use and today is the only one surviving in liturgy; d) the Sienese indiction, beginning on 8 September, and anticipating the year of the Circumcision by three months and twenty-three days; limited in use to the territory of Siena, it is always found in synchrony with the year ab incarnatione in the Florentine style.
Starting from the first documentation of the use of the indiction, it has been hypothetically calculated that the year 1 A.D. would correspond to the 4th indiction, i.e., that the beginning of the Christian era is three years behind the beginning of an indiction cycle: “Quarta indictione secundum Dionysium natus est Dominus” (Bede, De temporum ratione, 49: PL 90, 496). From this observation the rule for calculating the indiction was formulated, which is obtained by adding 3 units to the figure of the year of the Christian era and dividing the sum by 15: the remainder gives the indiction (a remainder of zero corresponds to the 15th indiction), while the quotient, which has no importance for chronological calculation, represents the hypothetical figure of the indiction cycles from the beginning of the common era to the date under consideration.