a line that indicates a boundary
"The surveyor used a red string to mark a borderline between the two neighboring properties."
A boundary or accepted division; a border.
"The new highway follows the borderline between the two counties, making it hard for drivers to tell where they are without checking their GPS."
To border, or border on; to be physically close or conceptually akin to.
"The suspicious activity was so intense that it bordered on criminal behavior."
of questionable or minimal quality
"borderline grades"
"marginal writing ability"
nearly; not clearly on one side or the other of a border or boundary, ambiguous.
"The line between his behavior being merely eccentric and actually dangerous was borderline, leaving everyone unsure if they should call for help."
nearly; not entirely but nevertheless to a great extent
"The patient's condition is borderline, showing severe symptoms that aren't quite enough for an official diagnosis yet."