A year with an inserted day — under a rule that is not simply 'every four years'.
A Gregorian leap year has 366 days. The rule is not every fourth year: it is every fourth year, except centuries, except centuries divisible by four hundred. 1900 was not a leap year; 2000 was.
The exception is the part software forgets, and it forgets it silently, because the next test of it is 2100 and nothing anybody is running will still be running.
The Julian calendar has no exception, which is the whole source of the drift Gregorian was created to fix. Other calendars intercalate on entirely different rules, and a leap year in one is not a leap year in another.
Leap year is one intercalation rule. Intercalation is the general idea, and a lunisolar leap MONTH is a different instance of it.
A leap day corrects the calendar against the orbit. A leap second corrects the clock against the rotation, and is not scheduled in advance.
See also intercalation · leap second · gregorian calendar · julian calendar
We hold this as computed, not stored — dates are materialised from rules per year. See what we hold.