Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 n. The period of time necessary to bring the solar calendar into harmony with the lunar calendar.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. the time (number of days) by which a solar year exceeds twelve lunar months; it is used in the calculation of the date of Easter
from the GNU version of the Collaborative International Dictionary of English
 n. 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.
from The Century Dictionary and Cyclopedia
 n. The excess of a solar over a lunar year or month. Hence, usually A number attached to a year by a rule of the calendar to show the age, in days completed and commenced, of the calendar moon at the beginning of the year—that is, on January 1st in the Gregorian, Victorian, and early Latin calendars, or March 22d in the Dionysian calendar, or old style.
Etymologies
Examples

Osiris, and winning from her the five days of the epact, which were added to complete the 365 days of the year.

In Table III, under 9, and in the line C, we find the epact
Encyclopaedia Britannica, 11th Edition, Volume 4, Part 4 "Bulgaria" to "Calgary"

When P = 2 the new moon falls on the ninth, and the epact is consequently twentytwo; and, in general, when P becomes 1 + x, E becomes 23  x, therefore P + E = 1 + x + 23  x = 24, and
Encyclopaedia Britannica, 11th Edition, Volume 4, Part 4 "Bulgaria" to "Calgary"

The epact 19 '(also distinguished by an accent or different character) is placed in the same line with 20 at the 31st of December.
Encyclopaedia Britannica, 11th Edition, Volume 4, Part 4 "Bulgaria" to "Calgary"

The epact of the year, therefore, or 19, must stand beside that day, whereas, according to the regular order, the epact corresponding to the
Encyclopaedia Britannica, 11th Edition, Volume 4, Part 4 "Bulgaria" to "Calgary"

In the calendar this epact first occurs before the 2nd of December at the 26th of November.
Encyclopaedia Britannica, 11th Edition, Volume 4, Part 4 "Bulgaria" to "Calgary"

On account of the solar equation S, the epact J must be diminished by unity every centesimal year, excepting always the fourth.
Encyclopaedia Britannica, 11th Edition, Volume 4, Part 4 "Bulgaria" to "Calgary"

The 2nd of January is therefore the day [v. 04 p. 0996] of the new moon, which is indicated by the epact twentynine.
Encyclopaedia Britannica, 11th Edition, Volume 4, Part 4 "Bulgaria" to "Calgary"

When the epact of the year is known, the days on which the new moons occur throughout the whole year are shown by Table IV., which is called the
Encyclopaedia Britannica, 11th Edition, Volume 4, Part 4 "Bulgaria" to "Calgary"

In consequence of the solar and lunar equations, it is evident that the epact or moon's age at the beginning of the year, must, in the course of centuries, have all different values from one to thirty inclusive, corresponding to the days in a full lunar month.
Encyclopaedia Britannica, 11th Edition, Volume 4, Part 4 "Bulgaria" to "Calgary"
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seanahan commented on the word epact
What is this used for? It seems quite useless.
September 26, 2008
artoparts commented on the word epact
To find the Epact: Divide the given year by 19; multiply the remainder by 11, and the product will be the Epact, if it does not exceed 29; but if it does, divide the product by 30, and the last remainder will be the Epact.
Example: Epact for 1846. 1846, divided by 19, gives 97 for the quotient, and 3 for the remainder, which is multiplied by 11, gives 33; this, divided by 30, gives the quotient 1, and the remainder 3; which is the Epact for the year 1846.
www.fer3.com/arc
See Golden Number for a similar formula for finding the Epact.
September 24, 2008
thebighenry commented on the word epact
epact (n): an electronic international treaty
May 15, 2008
whichbe commented on the word epact
A period added to harmonize the lunar with the solar calendar.
May 12, 2008
fbharjo commented on the word epact
epact 11 days that never were
January 14, 2007