Pith is the white, stringy, often bitter layer present in all citrus fruits between the outer peel and the interior pulp. It is known botanically as the mesocarp.
Pith is nearly always removed from the fruit before use, because it can impart a very bitter flavor to the final dish. One exception is the citron, which has been for a large and less bitter pith, and is used for candied citrus peel.