1. soil taxonomy the arbitrary depths of soil material within which certain diagnostic horizons, features, and other characteristics are used to classify soils —note the thickness is specific for each characteristic being considered but may be different for different characteristics 2. hydraulics a constriction or obstruction used in design of hydraulic structures, such as spillways or grade stabilization structures, at which depths upstream are subcritical and depths downstream are supercritical This definition last updated 10/25/2008