A leap year.
Of or relating to a leap year or to the extra day falling in a leap year.
Bissextile or leap year is a year containing one or more extra days (or, in the case of lunisolar calendars, an extra month) in order to keep the calendar year synchronized with the astronomical or seasonal year.
Word origin: Late Latin bi(s)sextilis (annus) leap year, equiv. to bissext(us).