transition zone

(fluid mechanics) Those conditions of fluid flow in which the nature of the flow is changing from laminar to turbulent.
(geology) A region within the upper mantle bordering the lower mantle, at a depth of 246-600 miles (410-1000 kilometers), characterized by a rapid increase in density of about 20% and an increase in seismic wave velocities. A region within the outer core, transitional to the inner core.