Definitions

from The American Heritage® Dictionary of the English Language, 4th Edition

  • adj. Of or relating to a leap year.
  • adj. Of or relating to the extra day falling in a leap year.
  • n. A leap year.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • adj. Having an extra day (of a leap year).
  • n. A leap year; A year having an extra day.

from the GNU version of the Collaborative International Dictionary of English

  • adj. Pertaining to leap year.
  • n. Leap year; every fourth year, in which a day is added to the month of February on account of the excess of the tropical year (365 d. 5 h. 48 m. 46 s.) above 365 days. But one day added every four years is equivalent to six hours each year, which is 11 m. 14 s. more than the excess of the real year. Hence, it is necessary to suppress the bissextile day at the end of every century which is not divisible by 400, while it is retained at the end of those which are divisible by 400.

from The Century Dictionary and Cyclopedia

  • Containing the bissextus or intercalary day: applied to those years which have 366 days, the extra day being inserted in the month of February. See bissextus.
  • n. A leap-year (which see).

Etymologies

Late Latin bissextīlis, containing an intercalary day, from bisextus (diēs), bis sextus (diēs) and Latin bisextum, intercalary day : bis, twice; see bis + sextus, sixth (because the sixth day before the Calends of March on February 24 occurred twice every leap year); see sext.
(American Heritage® Dictionary of the English Language, Fourth Edition)
From Latin bisextilis ("containing an intercalary day"), from bisextus ("intercalary day"), from bis- ("two") + sextus ("sixth"). In leap years there were two sixth days before March in the Roman calendar, thus the succession of days was ... 7 - 6 - 6* - 5 ... (Wiktionary)

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