WordNet (r) 3.0 (2006):

borderline
    adj 1: of questionable or minimal quality; "borderline grades";
           "marginal writing ability" [syn: borderline,
           marginal]
    n 1: a line that indicates a boundary [syn: boundary line,
         border, borderline, delimitation, mete]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "borderline":
   ambiguous, bordering, boundary, bounding, coastal, determinant,
   determinative, determining, dubitable, equivocal, extreme,
   fringing, frontier, limbic, liminal, limit, limiting, littoral,
   marginal, open, problematic, rimming, skirting, terminal,
   threshold, uncertain, unclear, undecided, unsettled