plumb line

1. A line from which a weight is suspended to determine verticality or depth.
2. A line regarded as directed exactly toward the earth's center of gravity.

A line with a plumb attached to it, used for finding the depth of water or determining the vertical on an upright surface.

See the Introduction, Abbreviations and Pronunciation for further details.

A cord or line, having a metal bob or weight attached to one end, which indicates the true vertical direction.

The noun has one meaning:

Meaning #1: a cord from which a metal weight is suspended pointing directly to the earth's center of gravity; used to determine the vertical from a given point
  Synonym: perpendicular