The year of the Gregorian calendar, common years having 365 days and leap years 366 days.
Each year exactly divisible by 4 is a leap year, except century years (1800, 1900, etc.), which must be exactly divisible by 400 (2000, 2400, etc.) to be leap years. The calendar year is based on the tropical year. Also called civil year.
This article is based on NASA's Dictionary of Technical Terms for Aerospace Use