- (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.
- a year containing an extra day or extra month in any calendar.
- 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
