A nebula was a cloud of cosmic matter. The Crab Nebula was once a star. (TV: Colony in Space) A spiral nebula was a gas cloud coalescing to form a star system. (TV: Underworld) Plasma storms could form in nebulae like the Horsehead Nebula. (TV: World War Three)
Some nebulae had habitable planets like the Nebula of Cyclops, the Dragon Nebula, and the Scorpion Nebula. (TV: The Brain of Morbius, Warriors of Kudlak, Planet of the Dead)
As the Twelfth Doctor was trying to find Clara Oswald in deep space, after having left her there, he asked her if she could see a nebula in a sort of wing-shape and green at the end. This helped him materialise the TARDIS around her. (TV: The Girl Who Died)