epact

[ee-pakt]
noun
  1. the difference in days between a solar year and a lunar year.
  2. the number of days since the new moon at the beginning of the calendar year, January 1.

Origin of epact

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
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

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British Dictionary definitions for epact

epact

noun
  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

Word Origin for epact

C16: via Late Latin from Greek epaktē, from epagein to bring in, intercalate, from agein to lead
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for epact
n.

1550s, "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 French épacte (12c.), from Late Latin epacta "an intercalary day," from Greek epaktos, literally "brought in, inported," verbal adjective of epagein "to intercalate, add, bring forward," from epi "on" (see epi-) + agein "to bring, to lead" (see act (v.)).

Online Etymology Dictionary, © 2010 Douglas Harper