Indiction

From late Roman through medieval times the indiction cycle was a sequence of 15 years, originally connected to rates of agricultural and property taxation, which were revised at the start of each cycle.  The word indiction (used alone) normally refers to an individual year, so e.g. the 'eighth indiction ' denotes the eighth year of the indiction cycle. 

In 537 The Emperor Justinian I decreed [Novella XLVII] that all dated contracts and legal documents should include the indiction as well as the regnal year, the month and day, and the name of the serving consul. 

The earliest Roman imperial practice was to start each indiction on 23 September, the birthday of Octavian Augustus. By the time of Justinian I the start date had shifted to 1 September. In the west, once Byzantine imperial influence had diminished, the Papacy established a different practice, with papal indictions beginning at Christmas, 25 December. 

On coinage, dating by indiction features on some Byzantine issues (both gold and base metal) struck during the sixth to eighth centuries. The practice was also briefly taken over by the Arabs, and a number of early Arab coin types from North Africa and Spain note the indiction. Centuries later, the custom was unexpectedly and fleetingly revived under the Paleologids.

The picture below shows an example of the earliest dated Byzantine type, a scarce follis of Justin I struck at Cyzicus in 525/6. The exergual text INSΔ abbreviates 'of the fourth indiction ' [indictionis (genitive)].