the difference in days between a solar year and a lunar year.
the number of days since the new moon at the beginning of the calendar year, January 1.

1545–55; < Late Latin epacta < Greek epaktḗ, noun use of feminine of epaktós added, equivalent to ep- ep- + ag(ein) to lead + -tos verbid suffix

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World English Dictionary
epact (ˈiːpækt)
1.  the difference in time, about 11 days, between the solar year and the lunar year
2.  the number of days between the beginning of the calendar year and the new moon immediately preceding this
3.  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]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

1552, "number of days by which the solar year exceeds a lunar one of 12 moons;" also "number of days into the moon on which the solar year begins;" from Fr. epacte, from L. epacta, from Gk. epaktos, verbal adj. of epagein "to intercalate, add, bring forward," from epi "on" + agein "to bring, to lead"
(cognate with L. agere "to drive, set in motion;" see act).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Abbreviations & Acronyms
[National] Energy Policy Act
The American Heritage® Abbreviations Dictionary, Third Edition
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