The Collaborative International Dictionary of English v.0.48:

Bissextile \Bis*sex"tile\, n. [L. bissextilis annus, fr.
   bissextus (bis + sextus sixth, fr. sex six) the sixth of the
   calends of March, or twenty-fourth day of February, which was
   reckoned twice every fourth year, by the intercalation of a
   day.]
   Leap year; every fourth year, in which a day is added to the
   month of February on account of the excess of the tropical
   year (365 d. 5 h. 48 m. 46 s.) above 365 days. But one day
   added every four years is equivalent to six hours each year,
   which is 11 m. 14 s. more than the excess of the real year.
   Hence, it is necessary to suppress the bissextile day at the
   end of every century which is not divisible by 400, while it
   is retained at the end of those which are divisible by 400.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Bissextile \Bis*sex"tile\, a.
   Pertaining to leap year.
   [1913 Webster]