Historically, the term "nebula" was used to describe any object, light or dark, that appeared as a definite object in the telescopes but lacked the sharp definition of a star or planet. Some were found to be galaxies, but the term is now generally used to describe clouds of interstellar dust or gas. Many are catalogued in the Messier catalog, such as M8, M16, M17 and M20 - these are now known as emission nebulae, glowing clouds of hot interstellar matter. Emission nebulae are some of the most interesting and beautiful objects in the sky.
Chaisson & McMillan