See borderline in Webster's New World Roget's A-Z Thesaurus II
noun
A fairly narrow line or space forming a boundary:
border, brim, brink, edge, edging, fringe, margin, periphery, rim, verge. (Chiefly Military) perimeter. See edgeA transitional interval beyond which some new action or different state of affairs is likely to begin or occur:
brink, edge, point, threshold, verge. See edge
adjective
Not affording certainty:
ambiguous, chancy, clouded, doubtful, dubious, dubitable, equivocal, inconclusive, indecisive, indeterminate, problematic, problematical, questionable, uncertain, unclear, unsure. (Informal) iffy. Idioms: at issue, in doubt, in question. See certain, clear
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