Plain English Meaning
The slowest speed at which the tires can start skimming on top of water on the runway instead of gripping the pavement.
Full Definition
The lowest groundspeed at which an aircraft tire can lose contact with a wet runway surface and ride on a thin film of water rather than on the pavement. For a tire in good condition, it is approximated by 9 times the square root of the tire pressure in pounds per square inch (9 × √PSI), giving the speed in knots.
Context Anchor
Seen when studying wet-runway landings, rejected takeoffs, and -distance decisions after or standing water is reported on the runway.