Polar night refers to the longer-than-24-hour nights that occur seasonally on the Earth's polar regions. Because the Earth's axis is tipped 26 degrees with respect to the plane of the elliptic, there are parts of the poles whose rotation path never gets exposed to light. The North and South Poles are the darkest areas of all, which each receive six months of continuous night and six months of continuous day. The depth of the polar night in the polar regions is when the lowest temperatures on Earth have ever been recorded, 89 degrees Celsius below zero, or negative 128 degrees Fahrenheit, measured at Vostok Station, Antarctica. Without heavy furs, this can lead to death in under two minutes.
The formal definition of the Arctic and Antarctic Circle has to do with polar night. Within the circle, there is at least one 24-hour period of complete darkness in the year, while outside it, darkness lasts less than 24 hours. Some far north and far south settlements experience very long nights, with only a few hours of sunshine throughout the day during winter. Examples include Hammerfest, Norway and research stations throughout most of Antarctica. Being deprived of light to this degree can be a risk factor for depression.
The inverse of the polar night is called the midnight sun, or polar day, where there is a prolonged day during the summer. This can make the environment (relatively) warm and permit for long treks over the ice, such as that achieved by Roald Amundsen, the Norwegian polar explorer who was the first to reach the South Pole, in 1911. With no night to speak of, travelers can set out whenever they want and have no freezing darkness to suffer through.
In some areas, instead of polar night, there is a polar twilight instead, where the Sun dips below the horizon, but by less than 10 degrees, bathing the entire landscape in a surreal and extended twilight. This light is sufficient for people to walk the streets and work, and some far north cities live with this twilight for months.
Some other planets, such as Pluto, have such a huge axial tilt that large percentages of their surface experience polar night and day. In the case of Pluto, this can lead to temperature as low as just 33 degrees Celsius above absolute zero.