Originating from a concern about the use of terms in hydroinformatics, this paper takes up the question of the nature of a definition and its consequences in a technical, and increasingly sociotechnical field like hydroinformatics. It is explained that the way things are defined is not so simple and so obvious as might at first appear, but is in fact an essentially ideological matter, having consequences not only within an individual discipline, but also for the applications of that discipline within a wider context. A mode of definition that satisfies the needs and expectations of hydroinformatics is then advanced.

The notion of a defining self is introduced in order to explain how the influences of the mode of definition attains to a greater influence in social and ecological applications. This influence is exerted through communication, which is necessarily realised through the use of tokens. The four classes of tokens necessary for all processes of communication are introduced, delineated and defined.

The way in which such broad concepts as ‘reality’ and ‘truth’ are defined is then seen to influence the definition of a model in the widest sense. Further definitions that are of central concern in hydroinformatics are developed accordingly. The paper concludes with a description of a phenomenon known only by its German name, as Gerede, that is at the root of much current confusion of terms in hydroinformatics, as in so many other places, and how its malign influences can be opposed.