The definition of usability has evolved over time. Some of the proposed definitions have been general, and apply to all types of interactive systems. Others were tremendously influenced by the experience of those who proposed the definitions and the domain in which they practiced usability. The definitions, in general, seem to agree that the targeted users, the complexity of the task, the type of technology (the interactive system), and environment (context of use) are the common factors that impact the usability of the interactive system. However, the attributes of usability that describe “a measure of how well actions are being performed with an interface” do seem to differ from one definition to another. It is precisely these attributes that we wish to focus on in this paper: how to identify them?, why they tend to differ from one definition to another?, why identifying them is crucial to the success of the interactive system?, and finally, how identifying the usability attribute can lead to effective user-centred development?