1. (in the Gregorian calendar) a year that contains 366 days, with February 29 as an additional day: occurring in years whose last two digits are evenly divisible by four, except for centenary years not divisible by 400.
2. a year containing an extra day in any calendar. Cf. common year.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.