Simple Definition of plumb line
: a tool that consists of a small, heavy object attached to a string or rope and that is used especially to see if something (such as a wall) is perfectly vertical
Full Definition of plumb line
1 : a line (as of cord) that has at one end a weight (as a plumb bob) and is used especially to determine verticality
2 : a line directed to the center of gravity of the earth : a vertical line
First Known Use of plumb line
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