Bissextile; a year containing 366 days; every fourth year which leaps over a day more than a common year, giving to February twenty-nine days. See Bissextile.
A year that has 366 days in it instead of 365. An extra day is added to the month of February for no other reason except that it's our shortest month. A leap year happens every four years.
Calculate leap years under a Gregorian calendar as follows: If a year is evenly divisible by 4,it is a leap year unless it is also evenly divisible by 100. If a year is evenly divisible by 100,it is not a leap year unless it is also evenly divisible by 400.