The lateral mixing of ambient environmental air into the edges of mixed-layer convective thermals or cumulus clouds, leaving a large undiluted core in the middle of the thermal. While this term is closely related to lateral entrainment, the phrase “lateral entrainment” is often associated with idealizations where the ambient air is mixed quickly across the whole diameter of the plume, leaving no undiluted core. While the lateral entrainment model has been applied successfully to smokestack plumes of meters to tens of meters in diameter, it is not appropriate for mixed-layer thermals of order 1 km in diameter.