leap year

noun
1.
(in the Gregorian calendar) a year that contains 366 days, with February 29 as an additional day: occurring in years whose last two digits are evenly divisible by four, except for centenary years not divisible by 400.
2.
a year containing an extra day or extra month in any calendar.
Compare common year.


Origin:
1350–1400; Middle English lepe yere

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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Collins
World English Dictionary
leap year
 
n
a calendar year of 366 days, February 29 (leap day) being the additional day, that occurs every four years (those whose number is divisible by four) except for century years whose number is not divisible by 400. It offsets the difference between the length of the solar year (365.2422 days) and the calendar year of 365 days

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

leap year
M.E., from leap (v.) + year. So called from its causing fixed festival days to "leap" ahead one day in the week.
Online Etymology Dictionary, © 2010 Douglas Harper
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Encyclopedia Britannica
Encyclopedia

leap year

year containing some intercalary period, especially a Gregorian year having a 29th day of February instead of the standard 28 days. The astronomical year, the time taken for the Earth to complete its orbit around the Sun, is about 365.242 days, or, to a first approximation, 365.25 days. To account for the odd quarter day, an extra calendar day is added every four years, as was first done in 46 BC, with the establishment of the Julian calendar. Over many centuries, the difference between the approximate value 0.25 day and the more accurate 0.242 day accumulates significantly. In the Gregorian calendar now in general use, the discrepancy is adjusted by adding the extra day to only those century years exactly divisible by 400 (e.g., 1600, 2000). For still more precise reckoning, every year evenly divisible by 4,000 (i.e., 16,000, 24,000, etc.) may be a common (not leap) year.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
The additional day in a leap year keeps the calendar in sync with the seasons.
The formula is a true yield annualized calculation, which is also adjusted to reflect leap year, when necessary.
The program knows which years are leap years and will display a check-mark to denote a leap year.
Many published algorithms ignore the leap year cycle and treat each year as any other year.
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