A year is the orbital period of the Earth moving in its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by changes in weather, the hours of daylight, and consequently vegetation and fertility. In temperate and subpolar regions, generally four seasons are recognized: spring, summer, autumn and winter. In seasonal tropical and subtropical regions, the wet (rainy or monsoon) season and the dry season are generally recognized.
A calendar year is an approximation of the Earth's orbital period in a given calendar. The Gregorian calendar considers a calendar year to be either a common year of 365 days, or a leap year of 366 days (as does the Julian calendar). The average year length across the complete leap cycle of the Gregorian (modern) calendar is 365.2425 days. ISO 80000-3, in an informative (cf. normative) annex, proposes the symbol, a, (for Latin annus) to represent a year of either 365 or 366 days. In English, the abbreviations, y and yr, are used.
In astronomy, the Julian year is a unit of time, defined as exactly 365.25 days each of exactly 86400 SI seconds, totaling 31557600 seconds.
The word, year, is also used of periods loosely associated with but not strictly identical to either the astronomical or the calendar year, such as the seasonal year, the fiscal year or the academic year, etc. By extension, the term, year, can mean the orbital period of any planet: for example, a Martian year or Venusian year is the time in which Mars or, respectively, Venus completes its own orbit. The term can also be used in reference to any long period or cycle, such as the Great Year.