leap year 
noun
A year, occurring once every four years, which has 366 days including 29 February as an intercalary day.To compensate for this discrepancy, the leap year is omitted three times every four hundred years....
- On the advice of a Greek astronomer, Sosigenes, he then [thereafter] started the leap year system.
- I know leap years are roughly every four years, but I think there are some that aren't like that.
Origin
Late Middle English: probably from the fact that feast days after February in such a year fell two days later than in the previous year, rather than one day later as in other years, and could be said to have ‘leaped’ a day.