An indiction is any of the years in a 15-year cycle used to date medieval documents. Each year was numbered: first indiction, second indiction, etc. However, the cycles were not numbered, thus other information is needed to identify the specific year.

Indictions originally referred to an agricultural or land tax in late third-century Roman Egypt. These were originally in 5-year cycles beginning in 287, then in a non-cyclic series which reached number 26 by 318. But by 314 the 15-year cycle had appeared. The Chronicon Paschale (c. 630) assigned its first year to 312-313, whereas a Coptic document of 933 assigned its first year to 297-298, one cycle earlier. Both of these were years of the Alexandrian calendar whose first day was Thoth 1 on August 29 in years preceding common Julian years and August 30 in years preceding leap years, hence each straddled two Julian years.

The indiction was first used to date documents unrelated to tax collection in the mid-fourth century. By the late fourth century it was being used to date documents throughout the Mediterranean. In the Eastern Roman Empire outside of Egypt, the first day of its year was September 23, the birthday of Augustus. During the last half of the fifth century, probably 462, this shifted to September 1, where it remained throughout the rest of the Byzantine Empire. But in the western Mediterranean, its first day was September 24 according to Bede, or the following January 1, called the papal indiction.

For the historical January 1 to December 31 year following September 1, 23, or 24, add three to the Anno Domini year, then divide by 15. The remainder is the indiction, but if the remainder is zero, then the indiction is 15.