tachocline

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun astronomy A region of abrupt transition from rotation to convection in the interior of the sun