leap year

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A year in the Gregorian calendar having 366 days, with the extra day, February 29, intercalated to compensate for the quarter-day difference between an ordinary year and the astronomical year.
• noun An intercalary year in a calendar.

from The Century Dictionary.

• noun A year containing 366 days, or one day more than an ordinary year; a bissextile year. See bissextile.

from the GNU version of the Collaborative International Dictionary of English.

• noun Bissextile; a year containing 366 days; every fourth year which leaps over a day more than a common year, giving to February twenty-nine days. See bissextile.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun In the Gregorian calendar, a year having 366 days instead of the usual 365, with the extra day added to compensate for the fact that the Earth rotates approximately 365.25 times for each revolution it makes around the Sun.
• noun In the Jewish calendar or other lunisolar calendars, a year having 13 months instead of 12, with the extra month added because 19 solar years is approximately 19*12+7 lunar months.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun in the Gregorian calendar: any year divisible by 4 except centenary years divisible by 400