The Collaborative International Dictionary of English v.0.48:

Indiction \In*dic"tion\, n. [L. indictio: cf. F. indiction. See
   Indict, Indite.]
   [1913 Webster]
   1. Declaration; proclamation; public notice or appointment.
      [Obs.] "Indiction of a war." --Bacon.
      [1913 Webster]

            Secular princes did use to indict, or permit the
            indiction of, synods of bishops.      --Jer. Taylor.
      [1913 Webster]

   2. A cycle of fifteen years.
      [1913 Webster]

   Note: This mode of reckoning time is said to have been
         introduced by Constantine the Great, in connection with
         the payment of tribute. It was adopted at various times
         by the Greek emperors of Constantinople, the popes, and
         the parliaments of France. Through the influence of the
         popes, it was extensively used in the ecclesiastical
         chronology of the Middle Ages. The number of indictions
         was reckoned at first from 312 a. d., but since the
         twelfth century it has been reckoned from the birth of
         Christ. The papal indiction is the only one ever used
         at the present day. To find the indiction and year of
         the indiction by the first method, subtract 312 from
         the given year a. d., and divide by 15; by the second
         method, add 3 to the given year a. d., and the divide
         by 15. In either case, the quotient is the number of
         the current indiction, and the remainder the year of
         the indiction. See Cycle of indiction, under Cycle.
         [1913 Webster]

WordNet (r) 3.0 (2006):

indiction
    n 1: a 15-year cycle used as a chronological unit in ancient
         Rome and adopted in some medieval kingdoms

Bouvier's Law Dictionary, Revised 6th Ed (1856):

INDICTION, computation of time. An indiction contained a space of fifteen
years.
     2. It was used in dating at Rome and in England. It began at the
dismission of the Nicene council, A. D. 312. The first year was reckoned the
first of the first indiction, the second, the third, &c., till fifteen years
afterwards. The sixteenth year was the first year of the second indiction,
the thirty-first year was the first year of the third indiction, &c.