plumb line

from The American Heritage® Dictionary of the English Language, 5th Edition.

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

from The Century Dictionary.

• noun A cord or line to one end of which is attached a metal bob or weight, used to determine vertical direction, depth of water, etc.; a plummet.
• To measure, sound, or test by means of a plumb-line.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A cord with a weight attached, used to produce a vertical line.
• noun Any vertical reference line.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun 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