Nebula
Also called: nebulae.
A cloud of gas and dust in space — glowing on its own, reflecting starlight, or blocking the light behind it.
“Nebula” is Latin for mist, and once meant any fuzzy patch in a telescope (galaxies included). Today it means interstellar clouds of gas and dust. They come in several kinds: emission nebulae that glow like neon signs, reflection nebulae lit by nearby stars, dark nebulae seen in silhouette, and the shells cast off by dying stars — planetary nebulae and supernova remnants.
At the eyepiece
Most nebulae look grey to the eye, which can't see colour in faint light. A nebula filter can transform the emission kinds.