leap year Definition

leap year
noun

1. A year of 366 days, with an intercalary day on 29 February, which occurs once every four years when the year is divisible by 4 (eg 1996). For end-of-century years, the year must be divisible by 400, thus accommodating the extra approximately quarter-day per year that it takes the Earth to circle the Sun.

Etymology: 14c: possibly from the fact that any day falling after 29 February will not fall on the day of the week after the day it fell on in the previous year as is usual, but will leap over to the next day.