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A season is one of the major divisions of the year, generally based on yearly periodic changes in weather.
In temperate and polar regions generally four seasons are recognized: spring, summer, autumn (fall), and winter.
In some tropical and subtropical regions it is more common to speak of the rainy (or wet, or monsoon) season versus the dry season, as the amount of precipitation may vary more dramatically than the average temperature.
In other tropical areas a three-way division into hot, rainy and cool season is used. In some parts of the world, special "seasons" are loosely defined based upon important events such as a hurricane season, tornado season or a wildfire season.

The seasons result from the Earth's axis being tilted to its orbital plane; it deviates by an angle of approximately 23.44 degrees. Thus, at any given time during summer or winter, one part of the planet is more directly exposed to the rays of the Sun (see Fig. 1). This exposure alternates as the Earth revolves in its orbit. At any given time, regardless of season, the northern and southern hemispheres experience opposite seasons (see Fig. 2 and Month ranges of seasons (below) and Effect of sun angle on climate).
Seasonal weather fluctuations also depend on factors such as proximity to oceans or other large bodies of water, currents in those oceans, El Niño/ENSO and other oceanic cycles, and prevailing winds.
In the temperate and polar regions, seasons are marked by changes in the amount of sunlight, which in turn often causes cycles of dormancy in plants and hibernation in animals. These effects vary with latitude, and with proximity to bodies of water. For example, the South Pole is in the middle of the continent of Antarctica, and therefore a considerable distance from the moderating influence of the southern oceans. The North Pole is in the Arctic Ocean, and thus its temperature extremes are buffered by the presence of all that water. The result is that the South Pole is consistently colder during the southern winter than the North Pole during the northern winter.
The cycle of seasons in the polar and temperate zones of one hemisphere is opposite to that in the other. When it is summer in the Northern hemisphere, it is winter in the Southern hemisphere, and vice versa, and when it is spring in the Northern hemisphere it is autumn in the Southern hemisphere, and vice versa.
In the tropics, there is no noticeable change in the amount of sunlight. However, many regions (famously the northern Indian Ocean) are subject to monsoon rain and wind cycles. Curiously, a study of temperature records over the past 300 years (David Thomson, Science, April 1995) shows that the climatic seasons, and thus the seasonal year, are governed by the anomalistic year rather than the tropical year.
In meteorological terms, the winter solstice and summer solstice (or the date maximum/minimum insolation) do not fall in the middle of winter and summer respectively. The heights of these seasons occur up to a month later due to seasonal lag. Seasons though, are not always defined in meteorological terms; see reckoning
Compared to axial tilt, other factors contribute little to seasonal temperature changes. It's a common misconception that the seasons are the result of the variation in Earth's distance to the sun due to its elliptical orbit.

In astronomical reckoning, the seasons begin at the solstices and equinoxes. The cross-quarter days are considered seasonal midpoints. The length of these seasons is not uniform because of the elliptical orbit of the earth and its different speeds along that orbit (see Kepler's laws).
In the conventional United States calendar:
Because of the differences in the Northern and Southern Hemispheres (see Meteorological below), it is no longer considered appropriate to use the old northern-seasonal designations for the astronomical quarter days. The modern convention for them is:

Meteorological seasons are reckoned by temperature, with summer being the hottest quarter of the year, and winter the coldest quarter of the year.
Using this reckoning, the Ancient Roman calendar began the year and the spring season on the first of March, with each season occupying three months. This reckoning is also used in Denmark, the former USSR, and Australia. In modern United Kingdom and Ireland there are no hard and fast rules about seasons, and informally many people use this reckoning.
So, in meteorology for the Northern hemisphere:
Conversely, for the Southern hemisphere:

Traditional seasons are reckoned by insolation, with summer being the quarter of the year with the greatest insolation, and winter the quarter with the least. These seasons begin about 4 weeks earlier than the Meteorological seasons, and 7 weeks earlier than the Astronomical seasons.
In Traditional reckoning, the seasons begin at the cross-quarter days. The solstices and equinoxes are the midpoints of these seasons. For example, the days of greatest and least insolation are considered the "midsummer" and "midwinter" respectively.
This reckoning is used by various traditional cultures in the Northern Hemisphere, including East Asian and Irish cultures.
So, according to Traditional reckoning,
And, the middle of each season is considered,

In Australia, the aboriginal people defined the seasons by what was happening to the plants, animals and weather around them. This led to each separate tribal group have different seasons, some with up to 8 seasons a year. However, most modern Aboriginal Australians follow the Meteorological Seasons, as is conventional amongst non-Aboriginal Australians.

In hemiboreal and temperate climates:
In winter, the plant can't hold the leaves without Chloryphyll.
In spring, the plants produce Chloryphyll and start to grow again.
In summer, the plants grow. Usually at this time the plants completely mature.
In autumn, the trees stop making Chloryphyll and turn yellow or shades of orange and red then drop their leaves.



Temperate: Spring · Summer · Autumn/Fall · Winter Tropical: Dry season · Wet season
Winter (89 days) begins on 21-22 Dec, the winter solstice
Spring (92 days) on 20-21 Mar, the spring equinox
Summer (93 days) on 20-21 June, the summer solstice
Autumn (90 days) on 22-23 Sept, the autumn equinox
The March Equinox
The June Solstice
The September Equinox
The December Solstice
spring begins on March 1,
summer on June 1,
autumn on September 1, and
winter on December 1.
summer begins on December 1,
autumn on March 1,
winter on June 1, and
spring on September 1.
Winter begins on 5-10 Nov, Samhain, 立冬 (lìdōng),
Spring on 2-7 Feb, Imbolc, 立春 (lìchūn),
Summer on 4-10 May, Beltane, 立夏 (lìxià), and
Autumn on 3-10 Aug, Lughnasadh, 立秋 (lìqiū).
Mid-winter: 20-23 Dec, winter solstice, 冬至 (dōngzhì)
Mid-spring: 19-22 Mar, spring equinox, 春分 (chūnfēn)
Mid-summer: 19-23 June, summer solstice, 夏至 (xiàzhì)
Mid-autumn: 21-24 Sept, autumn equinox, 秋分 (qiūfēn)
Perennial tea ceremony
Sports season
Australian Weather and Seasons
When do the Seasons Begin? (from the Bad Astronomer)
Solstice does not signal season's start (from The Straight Dope)
Why the Earth has seasons article on h2g2.
Aboriginal seasons of Kakadu
Indigenous seasons (Australian Bureau of Meteorology)
Mt Stirling Seasons
The Lost Seasons
Melbourne's six seasons
The Lengths of the Seasons (numerical integration analysis)