The season with the shortest days and the lowest average temperatures, winter astronomically begins on the winter solstice (when the tilt of the Earth's axis is most oriented away from the Sun) and ends on the vernal equinox (when the tilt of the Earth's axis is oriented neither from nor to the Sun causing the Sun to be located vertically above a point on the equator). The winter solstice occurs between December 20 and December 23 in the Northern Hemisphere and between June 20 and June 23 in the Southern Hemisphere. The vernal equinox ending winter occurs on March 20 or 21 in the Northern Hemisphere and September 22 or 23 in the Southern Hemisphere.

Meteorological winter is the season having the shortest days and the lowest temperatures. The coldest average temperatures of winter are usually experienced in January in the Northern Hemisphere and in July in the Southern Hemisphere. In other words, New York City experiences winter in January and Buenos Aires in July.

The lowest winter temperature ever recorded was -128.6 F on July 21, 1983 at the Russian research station located near the South Pole at Vostok, Antarctica. In the Northern Hemisphere, the Siberian towns of Verkhoyansk and Omyakon frequently record -90 F in January and February. Please note that all of these frigid temperatures do not include a wind chill factor.

Traditional winter begins and ends in many nations and cultures differently. In Celtic nations such as Ireland using the Irish calendar and in Scandinavia, the winter solstice is traditionally considered as midwinter, with the winter season beginning November 1 on All Hallows or Samhain. Winter ends and spring begins on Imbolc or Candlemas, which is February 1 or February 2. This system of seasons is based on the length of days exclusively. The three-month period of the shortest days and weakest solar radiation occurs during November, December and January in the Northern Hemisphere and May-July in the Southern Hemisphere. Also many mainland European countries tend to recognize St. Martin's day (November 11) as the first calendar day of winter. In Chinese astronomy (and other East Asian calendars) winter is taken to commence around November 7.

In ancient China, winter was known as Black Tortoise.