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.
the age of the Moon: the number of days since new moon, diminished by one day, on January 1 in the Gregorian ecclesiastical lunar cycle. (See Gregorian calendar: lunar phases.)
(From Greek épaktai hemérai, meaning 'additional days') The moon's age on a given day of the solar year, frequently used to give the current lunation's age on the first day of a new year.
(From Greek épaktai hemérai) The surplus number of days in a solar year over those in a lunar year. It indicates the current lunation's age on the first day of a new year.
The age of the moon on 1st January in any year; when this is known its age at any subsequent date in the year can be deduced. Since twelve lunations equal 354-36 days and a year equals 365.24 days, the epact increases by a little under eleven days from year to year, with a backward step of 29 1/2 days each time it passes this figure
The epact (from Greek: epaktai hèmerai = added days) is, as the second Canon of the Gregorian calendar reform puts it, "nothing else than the number of days which the common solar year of 365 days surpasses the common lunar year of 354 days" (Latin: Epacta nihil aliud est quam numerus dierum quibus annus solaris communis dierum 365 annum communem lunarem dierum 354 superat).