leap year

• leap-year

From Middle English lepe yeer, lep ȝere, lyp ȝere, equivalent to leap +‎ year. Probably from a much older formation related to the Old Norse hlaup-ár (“leap year”), Old English mōnan hlȳp (“moon's leap”) (compare Latin saltus lunae used in reckoning lunar months on a 19-year cycle). The name is presumably related to the phenomenon that any fixed date of a 365-day calendar advances one weekday each year but every date of a 366-day year (on modern reckonings after February 29) advances by two weekdays instead.[1] For example, Christmas (December 25) fell on a Saturday in 2004, a Sunday in 2005, a Monday in 2006, and a Tuesday in 2007 but then "leapt" over Wednesday to fall on a Thursday in 2008.

leap year (plural leap years)

1. A year in the Julian or Gregorian calendars with an intercalary day added to February, used to adjust for the extra hours of the solar year.
   • 1878, W.S.B. Woolhouse & al., "Calendar" in the Encyclopædia Britannica, 9th ed., Vol. IV, p. 666–7:

     The additional day which occurred every fourth year [after the Julian Reform] was given to February, as being the shortest month, and was inserted in the calendar between the 24th and 25th day. February having then twenty-nine days, the 25th was the 6th of the calends of March, sexto calendas; the preceding, which was the additional or intercalary day, was called bis-sexto calendas,—hence the term bissextile, which is still employed to distinguish the year of 366 days. The English denomination of leap-year would have been more appropriate if that year had differed from common years in defect, and contained only 364 days. In the ecclesiastical calendar the intercalary day is still placed between the 24th and 25th of February; in the civil calendar it is the 29th.

2. (inexact) Any other year featuring intercalation, such as a year in a lunisolar calendar with 13 months instead of 12, used to maintain its alignment with the seasons of the solar year.

• (Julian & Gregorian): bissextile, bissext

• leap day
• leap second

• common year