The Collaborative International Dictionary
Monthly epact
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.