The Whitcomb area rule, named after NACA engineer Richard Whitcomb and also called the transonic area rule, is a design procedure used to reduce an aircraft's drag at transonic speeds which occur between about Mach 0.75 and 1.2. Wikipedia

Rule in physics, relating to an aircraft's drag at transonic or supersonic speed Wikidata

A rule stating that an airplane designed with the same cross-sectional area distribution in the longitudinal direction as the Sears-Haack body generates the same wave drag as this body, largely independent of the actual shape; used in aerodynamics to reduce drag, typically by narrowing the fuselage at the wing roots. Wiktionary