(1) The sudden and usually turbulent passage of water in an open channel, under conditions of free flow, from a flow depth below critical depth to flow depth above critical depth; during this passage the velocity changes from supercritical to subcritical. It represents the limiting condition of the surface curve, in which that curve tends to become perpendicular to the stream bed. (2) In a closed conduit, the sudden rise from part-full flow at a supercritical velocity to full flow under pressure; the depth plus the pressure head downstream from the hydraulic jump equals the high stage obtained for open-channel flow. (3) A hydraulic method for dissipating energy in an open channel, in a sewer, or at the toe of a spillway section of a dam. (4) A hydraulic method to promote turbulence. (5) An abrupt rise in water surface which may occur in an open channel when water flowing at a high velocity is retarded.