Nomenclature surrounding drug safety needs to be clear and unambiguous, so that patients, prescribers, manufacturers, and regulators can all understand each other. In particular, it needs to make it clear how adverse events and drug therapy are related to one another, how they are best classified, and their frequency, intensity and seriousness. In this article, we therefore discuss and define terms used in the field of drug safety, particularly terms that are sometimes misunderstood or misused, including medicinal product, pharmaceutical formulation, excipient, adverse event (or experience) and adverse drug reaction (or effect). We also discuss terms used to define the seriousness, intensity, and risk of adverse reactions, and their classification. Instead of creating definitions from scratch, as is commonly done, we have taken the novel approach of critically examining definitions that have been proposed or widely used and have formulated new or modified definitions based on a logical appraisal of their merits and demerits. We hope that these definitions will lead to discussion that will allow a corpus of satisfactory definitions to be widely agreed.