From AMS Glossary
(Also called slab models.) An approximation that treats the atmospheric boundary layer as though variables such as potential temperature, momentum, pollutants, and humidity were uniform with height.
This type of model is popular because of its simplicity, requiring forecasts of only the average variables in the mixed layer and of the change of mixed-layer depth. During sunny days over land, the actual boundary layer is often sufficiently well mixed that a uniform slab approximation is a fairly good approximation.