Epact, a name given to the excess of the solar month over the lunar, amounting to 1 day 11 hours 11 minutes and 57 seconds, and of the solar year over the lunar amounting to 11 days.

E"pact (ē"păkt), n. [F. épacte, 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.