Usability

Usability is how easily and reliably people can use a product to do what they came to do, ideally with little effort, few errors and no need for instructions. A highly usable checkout, for example, lets a first-time visitor pay without pausing to work out how.

It is like a door with a handle that instantly tells you to push or pull, rather than one you have to fight. Usability matters because confusion costs sales and support time, and a product that feels obvious beats a cleverer one that people cannot readily figure out.