The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   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.
      [1913 Webster]