The definition is clearly found in the opening paragraph of Chapter 1. Here it is:
Definition of Medicine (Medical Pharmacology)
From Katzung's Basic and Clinical Pharmacology, 16th Edition, Chapter 1 — Introduction: The Nature of Drugs & Drug Development & Regulation:
"Medical pharmacology is often defined as the science of substances used to prevent, diagnose, and treat disease."
The chapter further contextualizes this by defining pharmacology broadly:
"Pharmacology can be defined as the study of substances that interact with living systems through chemical processes." These interactions usually occur by binding of the substance to regulatory molecules and activating or inhibiting normal body processes.
Additionally, Toxicology is described as the branch of pharmacology that deals with the undesirable effects of chemicals on living systems — from individual cells to humans to complex ecosystems.
So in summary, Katzung defines medicine in the pharmacological context through the lens of medical pharmacology: the science of substances used to prevent, diagnose, and treat disease.
— Katzung's Basic and Clinical Pharmacology, 16th Edition, p. 13