Epact /(ē"păkt)/

E·pact

Epact

n.
  1. 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. (Chron.)

Phrases & Compounds
Annual epact
the excess of the solar year over the lunar year, -- being eleven days.
Menstrual epact
the excess of a calendar month over a lunar.