Meaning of fall line

fall' line"

Pronunciation: [key]
  1. an imaginary line, marked by waterfalls and rapids, where rivers descend abruptly from an upland to a lowland.
  2. (caps.)the imaginary line between the Piedmont and the Atlantic coastal plain.
  3. the path of natural descent from one point on a slope to another.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.