not clear synonym vague模糊的;不清楚的 a nebulous concept 模糊的概念
Oxford Collocations DictionaryNebulous is used with these nouns:- concept
Word Originlate Middle English (in the sense ‘cloudy’): from French nébuleux or Latin nebulosus, from nebula ‘mist’. The current sense dates from the early 19th cent.