une année bissextile - leap year

All leap years are numbers that are multiples of four, or divisible by four; thus the years 1768, 1772, 1776, etc. are leap years. 

Toutes les années bissextiles tombent sur des nombres multiples de quatre, ou divisibles par quatre; ainsi les années 1768, 1772 , 1776, etc. , sont bissextiles.

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[The etymology of this word, bissextile, is Latin. It comes from words meaning "twice" and "sixth," because the leap day was inserted six days before the first day of March.]