Something that is almost but not quite meeting a standard or requirement; the edge or frontier between two places or conditions.
Compound of 'border' (edge of an area) and 'line' (a mark), literally meaning 'the line at a border.' First used in the 1800s for geographic boundaries, then metaphorically for states between two conditions.
A borderline grade of 79% when 80% is passing feels incredibly unfair—you're on the line between two categories. This is why colleges care about 'borderline cases' and why one point can feel like everything.
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