Plain English Meaning
The slowest speed at which your tires can start skimming on top of standing water instead of rolling on the runway. Below this speed the tires stay in contact with the pavement; at or above it, water can get under the tire and lift it.
Full Definition
The lowest groundspeed at which a tire can begin dynamic hydroplaning on a wet runway. For a smooth, round tire, it is approximately 8.6 times the square root of the tire pressure in pounds per square inch (PSI). At or above this speed, standing water can lift the tire off the runway surface, causing loss of braking and steering effectiveness.
Context Anchor
Seen in takeoff and landing performance discussions for with standing water, especially when judging braking and after or during a .