A year with one extra day (February 29) and therefore 366 days long. The day is added to keep the vernal equinox on or around March 21. In the Julian calendar, every fourth year was a leap year. In the Gregorian calendar, century years are not leap years unless divisible by 400; hence three leap years are omitted in a 400-year period.
Subjects: Astronomy and Astrophysics.