Definition of "nebulosity"

With higher powers the field does not appear to be sufficiently large to allow of each streak of nebulosity being compared with the surrounding parts where the nebulosity is not quite so bright.

1868, Lord Oxmantown, An account of the observations on the great nebula in Orion

If one conceives by analogy all the stars formed in this manner, one can imagine their anterior state of nebulosity itself preceded by other stars in which the nebulous matter was more and more diffuse, the nucleus being less and less luminous and dense.

2007, Stephen Hawking, God Created The Integers: The Mathematical Breakthroughs that Changed History