noun

a boundary separating two countries or areas.

adjective

only just acceptable in quality or as belonging to a category.

references may be requested in borderline cases

marginal,indefinite,uncertain,unsure,unsettled,undecided,up in the air,doubtful,open to doubt,problematic,indeterminate,unclassifiable,ambivalent,equivocal,questionable,open to question,disputable,debatable,arguable,controversial,contentious,moot,iffy,dodgy