What exactly is addison's disease?

Addison's: In addison's, the adrenal gland is destroyed by your immune system. It is an autoimmune disease. Without the adrenal gland, critical hormones such as cortisol and aldosterone are lacking. Addison's can be lethal. Fortunately, if detected, it can be treated with replacement hormones.

Adrenal failure: Destruction or dysfunction of the adrenal cortex (from a variety of causes) leading to inadequate cortisol production. May be life-threatening as cortisol release is a critical part of the body's response to infection, inflammation and other stresses.