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Inverno é uma das quatro estações do ano. O inverno do hemisfério norte é chamado de "inverno boreal", e o do hemisfério sul é chamado de "inverno austral". O "inverno boreal" tem início com o solstício de inverno nohemisfério norte, que ocorre por volta de 21 de dezembro, e termina com o equinócio de primavera, que acontece perto de 21 de março nesse mesmo.

El invierno es una de las cuatro estaciones de las zonas templadas. La palabra invierno viene del latínhibernum. Esta estación se caracteriza por días más cortos, noches más largas y temperaturas más bajas a medida que nos alejamos del ecuador.
Astronómicamente, comienza con el solsticio de invierno alrededor del 21 de junio en el hemisferio sur y el 21 de diciembre en el hemisferio norte, termina con el equinoccio de primavera, alrededor del 21 de septiembre en el hemisferio sur y el 21 de marzo en el hemisferio norte (aunque las fechas varían según el año). El hecho de que la órbita de la tierra sea elíptica, se traduce en una duración menor en el hemisferio norte y mayor respecto a este en el hemisferio sur, ya que en Julio se produce el afelio, durante el invierno austral, y en Enero el perihelio durante el boreal. En resumen, el invierno dura aproximadamente 4 días más en el hemisferio austral respecto al boreal. Meteorológicamente, en cambio, se suelen considerar invernales los meses enteros de junio, julio y agosto en el hemisferio sur y diciembre, enero y febrero en el hemisferio norte.

Winter Astronomically, the winter solstice, being the day of the year which has fewest hours of daylight, ought to be the middle of the season, but temperature lag means that the coldest period normally follows the solstice, so the season is sometimes regarded (in the USA and England) as beginning at the solstice and ending on the following equinox. In the Northern Hemisphere, depending on the year, this corresponds to the period between 21 or 22 December and 20 or 21 March. In Scandinavia, winter traditionally begins on 14 October and ends on the last day of February.

 In many countries in the Southern Hemisphere, including Australia , New Zealand and South Africa, winter begins on 1 June and ends on 31 August. 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 1 November, on All Hallows, or Samhain. 

Winter ends and spring begins on Imbolc, orCandlemas, which is 1 or 2 February. 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 through July in the Southern Hemisphere.)

Also, many mainland European countries tend to recognize Martinmas, St. Martin's day (11 November), as the first calendar day of winter. The day falls at midpoint between the old Julian equinox and solstice dates. Also, Valentine's Day (14 February) is recognized by some countries as heralding the first rites of spring, such as flowers blooming.
In Chinese astronomy and other East Asian calendars, winter is taken to commence on or around 7 November, with the Jiéqì (known as 立冬 lì dōng—literally, "establishment of winter").
The three-month period associated with the coldest average temperatures typically begins somewhere in late November or early December in the Northern Hemisphere and lasts through late February or early March. This "thermological winter" is earlier than the solstice delimited definition, but later than the daylight (Celtic) definition. Depending on seasonal lag, this period will vary between climatic regions.