n. 25B6; noun the borderline between old and antique : DIVIDING LINE, divide, division, demarcation line, line, cut-off point; threshold, margin, border, boundary.
25B6; adjective borderline cases : MARGINAL, uncertain, indefinite, unsettled, undecided, doubtful, indeterminate, unclassifiable, equivocal; questionable, debatable, controversial, contentious, problematic; informal iffy.