nebulous

Concise Oxford English Dictionary © 2008 Oxford University Press:
nebulous /ˈnɛbjʊləs/

▶adjective

• 1 in the form of a cloud or haze; hazy.

• 2 (of a concept or idea) vague or ill-defined.

• 3 Astronomy relating to a nebula or nebulae.

– derivatives
nebulosity noun,
nebulously adverb,
nebulousness noun.

– origin ME (in the sense ‘cloudy’): from Fr. nébuleux or L. nebulosus, from nebula ‘mist’.