Consider the flow of water over a dam as shown in the figure above, The height of water at section (1)--(1) is small, as we move towards down stream, the height or depth of water increases rapidly over a short length of the channel.

This is because at section (1)--(1), the flow is shooting flow as the depth of water at section (1)--(1) is less than critical depth, shooting flow is an unstable type of flow and does not continue on downstream side.

Then this shooting will convert itself in to a streaming or tranquil flow and hence depth of water will increase, this sudden increase of depth of water is called as hydraulic jump.

It can also be defined as "The rise of water level, which takes place due to the transformation of the unstable shooting flow to the stable streaming flow".