bissextile
/bɪsˈsɛks.tɪl/Definitions
1. noun
A year that is divisible by 4 but not by 100, unless it is also divisible by 400, in which case it is a leap year.
“The bissextile year will have an extra day added to the month of February.”
2. adjective
Related to a year that is a leap year.
“The bissextile calendar system is used to determine the timing of holidays.”