Shading Loss
The disproportionate drop in total output caused by shade falling on part of a panel limiting the current of the series-connected cell string. Since in a series circuit the weakest cell sets the current, shading even a small part of the surface can cause a far greater loss in output.
In a panel the cells are connected in series, and in a series circuit the current is the same for every element. Because a shaded cell can produce less current it drags the whole string down to its own level; on top of that a reverse voltage builds across it and it heats up. This local heating is called a hot spot and it can leave permanent damage.
To limit these two problems, bypass diodes are placed in the junction box of the panels. The diode takes the shaded cell group out of circuit and lets the current flow around it. The output of that group is lost, but the rest of the panel keeps working and the hot-spot risk is reduced.
In an RV the sources of shade are known: the air conditioning unit, the ventilation vent, the antenna, the roof rack and the trees where you park. When laying out the panels, the shade path of these elements through the day must be considered. The same logic applies at large-scale solar plants as well; there the shade is also produced by a permanent soiling layer, and that is why regular cleaning is part of the production plan.
Context in which this term is used: RV automatic levelling guide