When is a runway considered to be wet or slippery for 135 or 121 landing performance considerations?
I also could not find a definition for wet or slippery by the FAA, only for a contaminated runway:

CONTAMINATED RUNWAY− A runway is considered contaminated whenever standing water, ice, snow, slush, frost in any form, heavy rubber, or other substances are present. A runway is contaminated with respect to rubber deposits or other friction-degrading substances when the average friction value for any 500-foot segment of the runway within the ALD fails below the recommended minimum friction level and the average friction value in the adjacent 500-foot segments falls below the maintenance planning friction level.

(FAR/AIM Pilot/Controller Glossary, emphasis mine)
My Boeing 737 FCTM does however define:

A slippery surface is any surface where the braking capability is less than
that on a dry surface. Therefore, a surface is considered “slippery” when it
is wet or contaminated with ice, standing water, slush, snow or any other
deposit that results in reduced braking capability.

(Boeing 737 NG FCTM 2.13 Ground Operations, emphasis mine)
This sounds quite similar to the FAA definition of contaminated runways, so I would assume the same definition applies to wet or slippery in §121 and §135 operations as well.
The FCTM further says:

Slippery Runway Landing Performance
Slippery/contaminated runway advisory information is based on an assumption of
uniform conditions over the entire runway. This means a uniform depth for
slush/standing water for a contaminated runway or a fixed braking coefficient for
a slippery runway. The data cannot cover all possible slippery/contaminated
runway combinations and does not consider factors such as rubber deposits or
heavily painted surfaces near the end of most runways. With these caveats in
mind, it is up to the operator to determine operating policies based on the training
and operating experience of their flight crews.

(Boeing 737 NG FCTM A.27 Landing Roll - Factors Affecting Landing distance)
So it seems to be (at least in part) up to the operator to provide clear definitions on when exactly a runway should be considered slippery.

The Flight Safety Foundation has collected some more definitions, although from Europe, not the US:

Dry Runway
The European Join Aviation Authorities (JAA) defines dry runway as &quot;one which is neither wet nor contaminated, and includes those paved runways which have been specially prepared with grooves or porous pavement and maintained to retain 'effectively dry' braking action event when moisture is present.&quot;
Damp Runway
JAA says that a runway is considered damp &quot;when the surface is not dry, but when the moisture on it does not give it a shiny appearance.&quot;
Wet Runway
JAA says that a runway is considered wet &quot;when the runway surface is covered with water, or equivalent, less than specified [for a contaminated runway] or when there is sufficient moisture on the runway surface to cause it to appear reflective, but without significant areas of standing water.&quot;
Contaminated Runway
JAA says that a runway is contaminated &quot;when more than 25 percent of the runway surface are (whether in isolated areas or not) within the required length and width being used is covered by the following:

&quot;Surface water more than 3.0 mm [millimeters] (0125 in [inch]) deep, or by slush or loose snow, equivalent to more than 3.0 mm (0.125 in) of water;

&quot;Snow which has been compressed into a solid mass which resists further compression and will hold together or break into lumps if picked up (compacted snow); or,

&quot;Ice, including wet ice.&quot;



(FSF ALAR Briefing Note 8.5 - Wet or Contaminated Runways)