The term nebula is currently used for a cloud of dust and gas, typically visible, and including star-forming regions and supernova remnants. The Rho Ophiuchi Cloud Complex is an example of the former and Crab Nebula of the latter.
This usage of the term is a modification of its original, more general meaning, an object in the sky that appears fuzzy rather than point-like or well-defined (as is the Moon) and not detectably moving against the celestial sphere (as are comets). In particular, galaxies and some star clusters (e.g., some open clusters and globular clusters) were referred to as nebulae. For example, Andromeda was called the Great Andromeda Nebula. This usage isn't entirely dead as its use in the names and contents of older catalogs that include galaxies, such as the Messier Catalog, which was created under the name, Catalogue of Nebulae and Star Clusters. The term's usage shifted as observations began to distinguish galaxies from other such objects, and as the nature of galaxies became understood.
The word is also used within some phrases that indicate particular kinds of "fuzzy" objects, such as planetary nebula and protoplanetary nebula.