BISSEXT, or Bissextus (Lat. bis, twice; sextus, sixth), the day intercalated by the Julian calendar in the February of every fourth year to make up the six hours by which the solar year was computed to exceed the year of 365 days. The day was inserted after the 24th of February, i.e. the 6th day before the calends (1st) of March; there was consequently, besides the sextus, or sixth before the calends, the bis-sextus or "second sixth," our 25th of February. In modern usage, with the exception of ecclesiastical calendars, the intercalary day is added for convenience at the end of the month and years in which February has 29 days are called "bissextile," or leap-years.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)