FAA AC 25-32
Wet Runway.
A runway is wet when it is neither dry nor contaminated. For purposes of condition reporting and airplane performance, a runway can be considered wet when more than 25 percent of the runway surface area (within the reported length and the width being used) is covered by any visible dampness or water that is 1⁄8 inch (3 mm) or less.
Note: A damp runway that meets this definition is considered wet, regardless of whether or not the surface appears reflective.