Usability is one of the characteristics that compose the multi-dimension concept of information systems' quality. Technology improvements enable the building of information systems to be used "any place, any time", through mobile handheld devices and wireless networks. Usability is threatened by the use of mobile-wireless information systems, especially because of the use of small devices with tiny screens, the difficulty to operate and the use during mobility. Therefore, there is a need to measure the usability characteristic, in order to define the level of quality of such information systems. This paper introduces the definitions of usability, and presents the definition of new metrics, defined in order to measure the quality level of usability of mobile-wireless information systems. These metrics were empirically validated using four different experiments, based on handheld devices.