Despite the evident benefits of having a clear, agreed definition on what constitutes a disaster, the word resists one single interpretation. This chapter explores the issues and difficulties around defining the parameters and characteristics of disaster. It looks at specific instances in which the word ‘disaster’ is used as a highly precise technical and legal term, carrying critical life-or-death and monetary consequences regarding its formal declaration. These examples include the protocols for non-governmental organisations, specifically the United Kingdom-based Disasters Emergency Committee (DEC), as contrasted with the United States of America’s legal process via the Stafford Act, under which individual states and tribal governments can make direct application to the President, at whose sole discretion a major disaster can be declared. The chapter goes on to examine changing models of disaster and their definitions (including transglobal definitions); issues with data; and disparities between academic disciplines and perspectives to illustrate the inherent complexities underlying the real world and conceptual considerations that arise when we ask the question ‘what is disaster’?