E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The excess of the solar over the lunar year, the former consisting of 365 days, and the latter of 354, or eleven days fewer. The epact of any year is the number of days from the last new moon of the old year to the 1st of the following January. (Greek, epactos, feminine epact, adscititious.)