Definition
Leap year
A Gregorian leap year has 366 days, including 29 February, according to a divisibility rule.
What it means
Most years have 365 days. A leap year adds one day to February. Test the year in this order: divisible by 400 means leap year; otherwise divisible by 100 means not a leap year; otherwise divisible by 4 means leap year.
Remember it
Divisible by 4: usually yes. By 100: no. By 400: yes again.
Example
The year 2400 is divisible by 400, so it is a leap year. The year 2100 is divisible by 100 but not by 400, so it is not a leap year, even though it is divisible by 4.
Watch for
This divisibility rule is for the Gregorian calendar; other calendars can use different leap-year rules.
Common mix-up
A year ending in 00 needs the 400 test, not just the 4 test.
Exact meaning
In the Gregorian calendar, a leap year has 366 days and includes 29 February. A year is a leap year if it is divisible by 4, except that a year divisible by 100 is not a leap year unless it is also divisible by 400.