leap year

plural leap years · alternative leapyear

1. A year in the Julian or Gregorian calendar with an intercalary day added to February (in the Gregorian calendar, February 29), used to adjust for the extra hours of the solar year; a 366-day year.

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   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 twenty-fourth and twenty-fifth day. February having then twenty-nine days, the twenty-fifth was the sixth 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.

   1842, “CALENDAR”, in The Encyclopædia Britannica or Dictionary of Arts, Sciences, and General Literature. […], 7th edition, volume VI, Edinburgh: Adam and Charles Black, →OCLC, page 5, column 2

   Synonyms: bissext, bissextile, bissextile year, intercalary year

2. broadlyAny 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.