intercalation
insertion of a leap day, week, or month into some calendar years to make the calendar follow the seasons or moon phases
leap second
extra second inserted or removed to keep civil time in sync with the Earth's rotation
leap week calendar
calendar system with a whole number of weeks every year, and with every year starting on the same weekday
intercalary month
month added to a calendar in leap years
leap day
additional day occurring in a leap year