If you only know usability from real-world usability, or more likely the lack of it (good usability is unobtrusive), here are the standard definitions:
“[Usability refers to] the extent to which a product can be used by specified users to achieve specified goals with effectiveness, efficiency and satisfaction in a specified context of use.” - ISO 9241-11For more information, see The User Experience Professionals’ Association website.
“Human-centered design is characterised by: the active involvement of users and a clear understanding of user and task requirements; an appropriate allocation of function between users and technology; the iteration of design solutions; multi-disciplinary design.” - ISO 13407
Of course I’ll also take the opportunity to mention my paper titled Software Usability and Legal Informatics (draft paper on SSRN) which I will be presenting later this month at the KnowRight conference. As far as I know, there has been very little earlier scholarship on the topic in legal informatics, but pointers are most welcome. I will be pursuing this line of research further in other articles at least over the next couple of years.
[Update: presentation now available here.]