indiction (n.) Look up indiction at
late 14c., "period of fifteen years," a chronological unit of the Romans, from Latin indictionem (nominative indictio) "declaration, appointment," noun of action from past participle stem of indicere "to declare publicly, proclaim, announce," from in- "in" (see in- (2)) + dicere "to speak, say, tell" (see diction).

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] and continued in use in the Middle Ages. The name refers to the "proclamation," at the beginning of each period, of the valuation upon which real property would be taxed.