The word usability arises in relation with the Human- Computer interface studies. The interface of a program or of a web site can be useful because it performs the whole range of operations specified, but can be of low usability, for example, due to a high complexity that makes it difficult to be used efficiently for non-trained people. V Visualisation Formation in the mind of a visual image of an abstract concept. Visualisation derives, in this context, from the graphic representation of variables associated to the concept that one wants to visualize. For instance, a plot of the fever vs. time allows us to visualize the evolution of the illness.
Illness is the concept.