tach·o·cline 
(t
ăk
ə-kl
īn
′)
n. The boundary in the solar interior between the outer convection zone and the inner radiative zone. This boundary arises from the fact that the convection zone rotates faster near the solar equator than it does near the solar poles, while the interior of the sun rotates equally at all latitudes.
[Greek takhos, speed + -CLINE.]