年，或称地球年、太阳年，是与地球在轨道上绕太阳公转有关事件再现之间的时间单位。将之扩展，可以适用于任何一颗行星：例如，一「火星年」是火星自己完整的运行绕太阳轨道一圈的时间。

一般而言，一年的长度取为太阳在天球上沿黄道从某一定标点再回到同一定标点所经历的时间间隔。由于所选取的定标点不同，年的定义也不同，通常有：

由1984年起，天文学上采用儒略年（不可和历法上的儒略历相混淆）作为统一的年的时间单位，规定为精确的365.25日，与回归年相比每3000年误差1日。

现代公历里，平年一年为365天，闰年一年为366天。

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 soil fertility. In temperate and subpolar regions around the globe, four seasons are generally recognized: spring, summer, autumn and winter. In tropical and subtropical regions several geographical sectors do not present defined seasons; but in the seasonal tropics, the annual wet and dry seasons are recognized and tracked.