year

1. The period during which the earth completes one revolution around the sun.

This has several interpretations, as follows. 1) Sidereal year: time of true revolution around the sun, that is, the time it takes the earth (as seen from the sun) to reappear at the same fixed star, equal to 365.2564 mean solar days or 365 days, 6 hours, 9 minutes, 10 seconds; 2) Tropical year (
also called mean solar year, ordinary year): the time measured from one vernal equinox to the next, that is, the apparent revolution of the sun through the zodiac, equal to 365.2422 mean solar days or 365 days, 5 hours, 48 minutes, 46 seconds (this is not constant but decreases by about 5 seconds in one thousand years.); and 3) Calendar year: fixed by the Gregorian calendar of 365 days in ordinary years and 366 days in leap years.
See day.

1. Any arbitrary 12-month period selected for a special purpose, such as the water year.
   
   See also climatological year, farmer's year, grower's year.

Any function of time f(t) is periodic with period t if f(t) = f(t + τ) for all times t, where τ is the smallest number for which this equality holds.

The time it takes the earth to complete one full orbit around the sun relative to the fixed stars.

The time that it takes the sun to travel from one vernal equinox to the next.

The time interval between 1 January and 31 December, inclusive, in the Gregorian calendar.