What is Fall line in Geography?

In Geographical terms, A fall line is the imaginary line between two parallel rivers, at the point where rivers plunge, or fall, at roughly the same elevation. The Fall Line marks the point where each river leaves back the uplands for the lowlands, and therefore the limit of its navigability. Because of this and availability of water power there, many important industrial centers have become established along the line, e.g., Baltimore, Philadelphia, Richmond.