Year (calendar)

A calendar period of twelve months, usually 365 days or 366 days in a leap year. Its length approximates the time Earth takes to complete one orbit around the Sun.

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Month: A calendar unit whose length ranges from 28 to 31 days in the Gregorian calendar. Twelve months make up a calendar year, though their lengths differ.
Gregorian calendar: A solar calendar introduced in 1582 that is now the most widely used civil calendar. Its rules define the length of most modern civil calendar years.
Roman calendar: The calendar system used in ancient Rome before and during the reforms that produced the Julian calendar. Roman reforms shaped the calendar-year system inherited by later European calendars.
Fiscal year: A twelve-month accounting period used for budgeting and financial reporting, not necessarily matching the calendar year. It can begin on a different date while retaining a twelve-month span.
Leap year: A calendar year with an extra day inserted to keep the calendar aligned with the seasons. Its added day makes some calendar years 366 days long.
Julian calendar: A solar calendar introduced by Julius Caesar that assigns 365 days to most years and 366 to every fourth year. Its simpler leap-year rule makes its average year longer than the Gregorian year.
Julian reform: Julius Caesar's reform of the Roman calendar, implemented in 46 BCE and establishing the Julian calendar. It established a regular leap-year cycle for calendar years.
Academic year: An annual period of instruction used by schools and universities, often divided into terms or semesters. Its dates follow educational schedules rather than January-to-December boundaries.
Leap day: An extra calendar day, usually February 29, inserted in a leap year. This specific day supplies the calendar year's extra day.
Tropical year: The average interval between successive passages of the Sun through the vernal equinox. The Gregorian year is designed to track this seasonal cycle.