Cortex

Cortex: The outer portion of an organ.

The outer portion of the adrenal gland is called the adrenal cortex; the outer portion of the cerebrum is the cerebral cortex (a key part of the brain); the outer part of the kidney is the renal cortex (it lies just beneath the capsule surrounding the kidney); etc.

The cortex is in contrast to the inner portion of the organ which is called the medulla. Thus, there is the adrenal medulla; the medulla oblongata of the brain; the renal medulla; etc.

The word "cortex" is Latin for the bark of a tree. The plural of cortex is cortices. The adjective is cortical.