Mobile computing is a form of human–computer interaction where a computer is expected to be transported during normal usage. Mobile computing has three aspects: mobile communication, mobile hardware, and mobile software. The first aspect addresses communication issues in ad-hoc and infrastructure networks as well as communication properties, protocols, data formats and concrete technologies. The second aspect focusses on the hardware, i.e. mobile devices or device components. The third aspect deals with the characteristics and requirements of mobile applications.
Reference: Wikipedia - Mobile Computing, retreived 2011-04-25
Questions tagged mobile should in one way or another reflect a particular aspect unique to mobile devices, as compared to digital screen design in general. Often these are found in size, geographical mobility, different sensors or input methods as well as different ways of consuming and using interfaces from mobile devices.