epact

E'PACT, n. [Gr. adscititious, to adduce or bring; to drive.]

In chronology, the excess of the solar month above the lunar synodical month, and of the solar year above the lunar year of twelve synodical months. The epacts then are annual or menstrual. Suppose the new moon to be on the first of January; the month of January containing 31 days, and the lunar month only 29 days, 12h. 44'3", the difference, or 1 day, 11h. 15'57", is the menstrual epact. The annual epact is nearly eleven days; the solar year being 365 days, and the lunar year 354.