Despite many attempts, the concept interdisciplinarity seems difficult to define, at least in a linguistic form. This has caused for some confusion of concepts and a lack of clarity regarding method in interdisciplinary science. However, tendencies toward e.g. problem-orientation, holistic approaches as wells as the emergence of new technologies make it relevant to examine how interdisciplinarity unfolds. Human-Centered Informatics (Humanistisk Datalogi) is a good and typical example of a field in which interdisciplinarity is a basic term. Drawing upon the semiotics of C.S. Peirce, I will present a conceptialisation of interdisciplinarity within Human-Centered Informatics. This conceptualisation is expressed through dividing science into a trichotomy of 'Domain', corresponding to Peirces notion of firstness, 'Analysis', corresponding to secondness, and 'Theory' corresponding to thirdness. With Richmonds [2005, 2006] idea of |>* (trichonic) and his vectorial analysis as a basis, interdisciplinarity is conceptualised as movements within this scientific triad. In this process, elements from other sciences are imported in the three different universes 'Domain', 'Analysis' and 'Theory'. I establish that this happens through some form of likeness in any of the three universes, and provide a simplified definition of what likeness means in this context. This conceptualisation of interdisciplinary science is a metaphor, according to which scientific elements 'move' from other sciences into Human-Centered Informatics. From this idea, I develop and present a model for interdisciplinarity, consisting of 6 different triadic operations which may come into play in interdisciplinary science. I provide examples of these 6 operations by applying them in an analysis in which I draw upon a broad selection of literature from the field of Human-Centered Informatics. I then proceed to derive the insights provided by the model, along these main lines: (1) The nature of the field Human-Centered Informatics, (2) The principle of tranfer of elements from other sciences, and its practical consequences and (3) The applicability of the model to other fields of science.