chalk line

from the GNU version of the Collaborative International Dictionary of English.

• noun a string impregnated with powdered chalk, used in construction to mark a straight line on a surface, such as a wall. The string is held taut on the surface between the two end points of the line to be made, the center of the line is drawn away from the surface, and the string is released and snapped against the surface, leaving a line of chalk in the desired location.
• noun the line of chalk on a surface left by a chalk line{1}.

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

• noun A line made on a surface by snapping a taut string or cord dusted with colored chalk.

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

• noun a chalked string used in the building trades to make a straight line on a vertical surface