超临界流是指流体的速度己超过波速，在气体动力学中类似的情形称为超音速。

波的信息以波速的速度传递，例如将小石头丢入水中．水中涟渏扩散的速度即为波速。流体速度则是当这个波扩散时，上面的叶子移动的速度。若小石头丢到超临界流中，涟渏会全部往下游方向扩散，若小石头丢到亚临界流中，涟渏会同时往上游及下游方向扩散。只有在超临界流中才会出现水跃的现象。

在流体力学，由两个行为之间的转变常会用无因次量来描述，而转变多半发生在此数字大于一或小于一的情形下。是否为超临界流可以用福禄数来判断。

其中

若，流体为亚临界流，若，流体为超临界流，若，流体为临界流。

A supercritical flow is a flow whose velocity is larger than the wave velocity. The analogous condition in gas dynamics is supersonic.

Information travels at the wave velocity. This is the velocity at which waves travel outwards from a pebble thrown into a lake. The flow velocity is the velocity at which a leaf in the flow travels. If a pebble is thrown into a supercritical flow then the ripples will all move down stream whereas in a subcritical flow some would travel up stream and some would travel down stream. It is only in supercritical flows that hydraulic jumps (bores) can occur. In fluid dynamics, the change from one behaviour to the other is often described by a dimensionless quantity, where the transition occurs whenever this number becomes less or more than one. One of these numbers is the Froude number: