adj.

1. Of or relating to a leap year.
2. Of or relating to the extra day falling in a leap year.

n.

A leap year.

[Late Latin bissextīlis, containing an intercalary day, from bisextus (diēs), bis sextus (diēs) and Latin bisextum, intercalary day : bis, twice; see bis + sextus, sixth (because the sixth day before the Calends of March on February 24 occurred twice every leap year); see sext.]

relating to the extra day in a leapyear (the second sixth day)