From AMS Glossary
The middle portion of the daytime atmospheric boundary layer characterized by convective thermals creating vigorous turbulence in a well-mixed region of vertically uniform potential temperature, wind speed, and pollutants.
This layer is above the radix and surface layers, and below the entrainment zone or capping inversion. Within it, smoke plumes are observed to loop up and down as they blow downwind. It is statically unstable, in spite of the adiabatic lapse rate.