A skill that requires identifying distinguishing features visually
Visual acuity is the player skill to visually identify, locate, or match distinguishing marks on puzzle pieces. This can include the identificatin of the pieces themselves, if those are melded into the environment.
Visual acuity is about testing the players sight. Can the player find the hidden image? Can they recognize an obscured pattern? Can they distinguish several similar items? Can they find the right viewpoint to see a morphed image?
Sight varies significantly between players. Increasingly small visual details can be hard for some players to recognize.
Some visuals can be hard to focus on, and thus the time required to view it should be limited. For example, placing a complex puzzle inside a stereogram image is ill-advised. But placing a single image or number is be fine.
Assume everything in this reference is a working draft, there's prone to be some mistakes and inconsistencies. I figure it's best to publish and get feedback rather than write for years in secret. The terms will change, the structure will shift, and the bugs will be chased out. It'll take a while.