From AMS Glossary
Smoke plumes that rise through the environment due to buoyancy or momentum become diluted with environmental air, where the rate of dilution is proportional to the rise rate of the plume. The entrainment coefficient is this constant of proportionality. This concept of lateral entrainment does not work well for convective thermals in the atmospheric boundary layer. One reason is that boundary layer thermals are part of a large overturning circulation involving the whole boundary layer during statically unstable conditions of free convection, rather than being thin currents of rising smokestack air.