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The period of time necessary to bring the solar calendar into harmony with the lunar calendar.
1. (Astronomy) the difference in time, about 11 days, between the solar year and the lunar year
2. (Astronomy) the number of days between the beginning of the calendar year and the new moon immediately preceding this
3. (Horology) the difference in time between the calendar month and the synodic month
[C16: via Late Latin from Greek epaktē, from epagein to bring in, intercalate, from agein to lead]
the difference in days between a solar year and a lunar year: a period of time added to a calendar to harmonize the two.
[1545–55; < Middle French epacte < Late Latin epactae < LGK epaktaí(hēmérai) intercalated (days), pl. of epaktós <epágein to bring in, intercalate]