a fiscal period of fifteen years used as a means of dating events and transactions in the Roman Empire and in the papal and some royal courts. The system was instituted by the Emperor Constantine in ad 313 and was used in some places until the 16th century.
[with numeral] a particular year in an indiction period.
from Latin indictio(n-), from the verb indicere(see indict)
indiction in other Oxford dictionaries
Definition of indiction in the US English dictionary