The definition of a nebula is a cloud of gas and dust in outer space.(noun)
A mass of dust in the sky that reflects light and appears as darkness in the sky may be an example of a nebula.
See nebula in Webster's New World College Dictionary
Origin: ModL < L, vapor, fog, mist < IE base *nebh-, moist, vapor, cloud > Gr nephos, nephelē, cloud, OE nifol, mist, darkness
See nebula in American Heritage Dictionary 4
noun pl. neb·u·lae (-lēˌ) or neb·u·las
Origin: Middle English nebule, cloud, mist
Origin: , from Latin nebula; see nebh- in Indo-European roots.
Learn more about nebula