Relating to a leap year, especially the calendar system that adds an extra day every four years to keep the calendar aligned with Earth's orbit.
From Latin bisextus (twice-sixth), referring to the doubling of the sixth day before the March calends in the Roman calendar. Medieval Latin developed bissextilis (leap year). The term evolved to describe any calendar system accounting for Earth's actual orbital period of approximately 365.25 days.
The word 'bissextile' reveals a quirk of human calendar-making: the Romans literally counted the same day twice (bis-sextus = twice-sixth) to add an extra day without renumbering everything—it's like they invented the leap year before they had the math to call it that!
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