grey area

Từ điển Oxford Advanced Learner 8th

ˌgrey ˈarea [grey area] BrE NAmE (especially BrE) (NAmE usually ˌgray ˈarea) noun
an area of a subject or situation that is not clear or does not fit into a particular group and is therefore difficult to define or deal with
•Exactly what can be called an offensive weapon is still a grey area.
•The new rules for police procedure cleared up a lot of grey areas.