What is Bissextile?

Legal Definition
The day which is added every fourth year to the month of February, in order to make the year agree with the course of the sun. It is called bissextile, because in the Roman calendar it was fixed on the sixth day before the calends of March, (which answers to the 24th day of February,) and this day was counted twice; the first was called bissextus prior, and the other bissextus posterior, but the latter was properly called bissextile or intersalary day. Although the name bissextile is still retained in its obsolete import, we intercalate the 29th of February every fourth Year, which is called leap year; and for still greater accuracy, make only one leap year out of every four centenary years. The years 1700 and 1800 were not leap years, nor will the .year A. D. 1900 be reckoned as one, but the year A. D. 2000 will be a leap year or bissextile. For a learned account of the Julian and Gregorian calendars, see Histoire du Calendrier Romain, by Mons. Blondel; also, Savigny Dr. Rom. §192; and Brunacci's Tract on Navigation, 275, 6. BLACK ACT, English law. An act of parliament made in the 9 Geo. II., which tears this name, to punish certain marauders who committed great outrages, in disguise, and with black faces. See Charlt. R. 166.
-- Bouviers Law Dictionary
Legal Definition
The day which is added every fourth year to the month of February, in order to make the year agree with the course of the sun. Leap year, consisting of 366 days, and happening every fourth year, by the addition of a day in the month of February, which in that year consists of twenty-nine days.
-- Black's Law Dictionary
Legal Definition
The day added to February in leap years.
-- Ballentine's Law Dictionary