indiction (n.)

late 14c., "period of fifteen years," a chronological unit of the Roman calendar that continued in use through the Middle Ages, from Latin indictionem (nominative indictio), literally "declaration, appointment," noun of action from past participle stem of indicere "to declare publicly, proclaim, announce," from in- "in" (from PIE root *en "in") + dicere "to speak, say, tell" (from PIE root *deik- "to show," also "pronounce solemnly").

Fixed by Constantine and reckoned from Sept. 1, 312. Originally for taxation purposes, it was "a common and convenient means for dating ordinary transactions" [Century Dictionary]. The name refers to the "proclamation," at the beginning of each period, of the valuation upon which real property would be taxed.