Viable Product - somewhat of a joke in our industry but a useful term to get what you can out the door. In his book 'Designing for Emotion', Aarron Walter applies the idea of Abraham Maslow’s hierarchy of needs to web design (which are fair bit can be applied to mobile), remapping the pyramid like so: Functional: the most fundamental ‘need’ of users is that the design works — that the print is good, the sign fits and the app works. Reliable: it’s no good if the design works once. For users to be able to trust and for the design to be successful, it needs to function well every time it’s needed. Usable: an all too often overlooked layer. Users have to be able to use the design for it to work. This could be sufficiently readable type size in a manual, intuitive navigation or having step free access to a station platform.