The sky is the part of the atmosphere or of outer space visible from the surface of any astronomical object. It is difficult to define precisely for several reasons. During daylight the sky of Earth has the appearance of a deep blue surface, as the result of the air's scattering of sunlight. The sky is sometimes defined as the denser gaseous zone of a planet's atmosphere. At night the sky has the appearance of a black surface or region scattered with stars.

During the day the Sun can be seen in the sky, unless covered by clouds. In the night sky (and to some extent during the day) the moon, planets and stars are visible in the sky. Some of the natural phenomena seen in the sky are clouds, rainbows, and aurorae. Lightning and precipitation can also be seen in the sky during storms. On Earth, birds, insects, aircraft, and kites are often considered to fly in the sky. As a result of human activities, smog during the day and light radiance during the night are often seen above large cities (see also light pollution).

In the field of astronomy, the sky is also called the celestial sphere. This is an imaginary dome where the sun, stars, planets, and the moon are seen to be travelling. The celestial sphere is divided into regions called constellations.

See skies of other planets for descriptions of the skies of various planets and moons in the solar system.

Sky+, or Sky Plus, is a personal video recorder (PVR) service for Sky Digital. Launched in September 2001, Sky+ allows the user to record, pause and instantly rewind live TV. The system performs these functions using an internal hard drive inside the Sky+ set top box. Its chief competitors in the UK market are the developing non-subscription PVR market, including Freeview Playback, and Virgin Media's V+, which has 3 tuners and a 160 GB hard drive. Sky reported in February 2008 that over 3 million households now have a Sky+ box.

The £10 per month subscription fee was discontinued for subscribers from 1 July 2007, but will continue for Freesat from Sky use.

Sky is a mystically-oriented children's science fantasy television serial made for ITV by HTV in 1975 and broadcast in six parts.

Looking something like a young, effete blond ubermensch, a morally ambiguous alien boy with weird solid blue eyes, the eponymous Sky (Marc Harrison), finds himself on Earth, where he uses his psychic powers for achieve his goal of ensuring a way back home. Sky finds the very world soul of Earth, in the form of nature, rejecting him, the way an immune system might an infection. In his quest to return home, he interlinks his own destiny with that of three human children. The serial was written by Bob Baker and Dave Martin, also known for their scripts for Doctor Who and a fantasy television series for children,Into the Labrynth.

Sky ( ) is a Malaysian-made Mandarin drama series, produced by and currently aired on 8TV, which combines newly-discovered celebrities, particularly from the station's reality programmes Malaysian Idol, Project Superstar Malaysia and I Wanna Be A Model, namely Hoon Mei Sim, Johnson Wee, Gary Yap, Candy Lim and Daniel Lee Chee Hun.

Sky began its run on 8TV on 6 May 2007 and ended its run after 13 weeks. The DVD release of the drama is scheduled to be released in August 2007. Currently, there is no word of any plans to distribute the drama internationally.

A young time-traveller with superhuman powers is stranded on Earth after running into a Black Hole. Pursued by the evil Goodchild, Sky is helped on his quest to find a way home by three human teenagers, Arby, Jane and Roy. Written by Anonymous