Originally "nebula" (Greek for "cloud"; plur. nebulae) was a catch-all name for any extended astronomical object, including galaxies beyond the Milky Way. This occasionally survives, but the stricter modern term is used for interstellar clouds of dust and gas, which are categorized by how they are illuminated.
The Triangulum emission nebula (NGC604) lies in a spiral arm of galaxy M33, 2.7 million light-years from Earth. This nebula ia a region in which stars are forming.
Some types of nebula have special names, such as dark nebulae, emission nebulae, planetary nebulae, reflection nebulae, and solar nebulae.