Ah, the wonders of wet paint.
The obvious definition is paint that has recently been applied to a surface and has not had time to dry and/or cure. Wet paint can sometimes have a thin skin and look dry but underneath is still working on drying. Jabbing a digit at it will result in a dimple and, possibly, a wrinkle if the finger moved parallel while poking the surface perpendicularly. Additionally, wet paint can smear off onto other things like new suits and expensive coats. This is why they came up with a small sign to warn others about the presence of wet paint.
For artists, they may have a canvas that has not dried. There's nothing worse than getting a masterpiece done and having someone coming over to view the work only to touch it to see if it is dry.
It's fascinating how the first instinct most folks have when they are told there is wet paint is they want to touch it to verify.
Iron Noder 2017