When a patient is brought into a trauma center in extremis, or becomes that way during a hospital stay, the intervention mantra is airway, breathing and circulation—the ABC's. We provide the nonspecialist physician, and those in the participating and supporting professions, a quick overview of the meaning of the testing terms in this environment. Many tests, added singly or in small groups over time as clinical and measurement technology evolved, but whose rationale may have become less clear, are defined and explained. Our format is simple—a brief overview of the physiologic environment and then the encyclopedic dictionary of the terminology. We include common terms as well as some of the confusing uses of terms besides lesser known facts influencing their clinical use on the basis of the analytical, medical, and managerial experience of the authors. Keep it in your lab coat pocket—you'll find it useful in understanding and teaching!