Source : Webster's Revised Unabridged Dictionary (1913)

Epact \E"pact\ ([=e]"p[a^]kt), n. [F. ['e]pacte, fr. Gr.
   'epakto`s brought on or in, added, fr. 'epa`gein to bring on
   or in; 'epi` on, in + 'a`gein to bring or lead. See {Epi-},
   and {Act}.] (Chron.)
   The moon's age at the beginning of the calendar year, or the
   number of days by which the last new moon has preceded the
   beginning of the year.

   {Annual epact}, the excess of the solar year over the lunar
      year, -- being eleven days.

   {Menstrual epact}, or {Monthly epact}, the excess of a
      calendar month over a lunar.