(1) Sudden difference in elevation in the bed of a stream of sufficient extent to cause the entire stream of water passing over it to drop nearly vertically for some distance before it resumes its course.
(2) Difference in elevation of the water surface between two points of a stream at a given time. (TR).
(3) Difference of water levels at the two sides of a weir or other hydraulic control structure.
(4) Drop of water level at a point of a stream during a certain time.
UNESCO/WMO International Glossary of Hydrology (2nd revised ed., 1992)