Dictionary.net

Every fourth year which leaps forward or adds one day in February, a year of 366 days.

Every fourth year, of 366 days; so called as, by adding a day to February, making the days thereafter leap forward one more than usual.

Every fourth year, which is made to consist of 366 days, February having in a leap-year 29 days; civil year, the year adopted by a nation for the computation of time; lunar year, the period of 12 lunar months, or 354 days; common year, a year of 365 days; Julian year, the year established by Julius Caesar, consisting of 365 days for three years, and 366 days for every fourth year; Gregorian year, the corrected Julian year, now adopted by most civilised nations; Sabbatic year, among the anc. Jews, every seventh year, during which the land was suffered to lie untilled; sidereal year, the time in which the sun, departing from the place of any fixed star, returns to the same position.