Steroid: A general class of chemical substances that are structurally related to one another and share the same chemical skeleton (a tetracyclic cyclopenta[a]phenanthrene skeleton).

Many hormones, body constituents, and drugs are steroids. Thus, the term steroid may carry many meanings. For example, steroids may refer to the corticosteroid drugs such as prednisone used to reduce swelling, pain, and other symptoms of inflammation. Vitamin D is a steroid, too, one that is necessary to normal health. The male hormone testosterone and its derivatives are steroids with anabolic effects that can be used medically (or illicitly) to build up muscle mass.

Cholesterol is yet again another steroid. In fact, the word steroid was coined in 1926 to refer to compounds like the sterols (as in cholesterol).