Sunlight was light given off by a star. The term was most often used to describe light from a planet's sun.
Sunlight occurred during the daytime, and an absence of sunlight on a planet's surface was known as nighttime. (PROSE: Dragonfire) For some planets, such as Earth, day and night alternated in a regular rhythm - for others this was not the case. Svartos was a planet that was tidally locked, with one side permanently in the sunlight of its nearby star and the other permanently in the dark. (PROSE: Dragonfire) Halergan 3 was a planet which for unknown reasons experienced constant sunlight, making it a popular holiday destination. (TV: The Ribos Operation)
Sunlight often made planets more habitable. Plants could produce oxygen from sunlight, which made it possible for life to exist on Earth. (AUDIO: Exploration Earth) Also on Earth, in 2018, Ramón Salamander used his sun store technology which guided sunlight to help crops grow, thus helping people get more food. (TV: The Enemy of the World) Sunlight helped produce food on other planets too - as part of a ruse to lure the Thals into their city on Skaro, the Daleks claimed to be able to grow vegetables using artificial sunlight. (TV: The Daleks)
However, sunlight was not always conducive to life. Some beings did not cope well with the conditions of sunlight. As one example, on Earth, the Sun's light was ultraviolet, which repelled Saturnyns. (TV: The Vampires of Venice) As another example, after taking the form of Mr Ring-A-Ding, Lux Imperator the God of Light was defeated by the Fifteenth Doctor and Belinda Chandra when the pair blew a hole in the wall of the Palazzo Movie Theatre, exposing him to so much light that he could no longer maintain a physical form. (TV: Lux) Finally, there were some types of sunlight which were hazardous to the majority of life forms: the planet Midnight orbited a sun which gave off X-tonic rays, a form of radiation which Professor Hobbes theorised would make the entire star system inhospitable to life. (TV: Midnight)
For many beings, the sensation of sunlight was pleasurable. (TV: Dalek) For humans, and possibly other species as well, sunlight was useful for its ability to produce a natural cosmetic tan. (PROSE: Sick Building)