Results based on the performance of common user tasks. (apple.com)
Usability is task and performance oriented, which means that it is focused on optimizing user task efficiency, and it is about removing problems in the User Interface. (elearningindustry.com)
D Richman in The Australian newspaper on 6 June, 1995 in an item called «Software savants take on routine user tasks», said: (slaw.ca)