What Is Usability?

Usability is at the heart of the user experience. Poor usability can lead users to abandon sites and it can even cost businesses thousands and thousands of dollars. Usability is measured in terms of a number of different dimensions; satisfy these and the user experience should improve in turn. Testing involving real users presents a relatively cheap approach to identifying and eradicating design problems. The gains associated with a user-centred approach can prove enormous for businesses, as it might be the difference between users sticking with a device/system or abandoning it in favour of a competitor.

via User Experience.