Usability is the degree to which something - software, hardware or anything else - is easy to use and a good fit for the people who use it.
Usability as a professional discipline
Usability, which is a dimension of User Experience practice, is an approach to product development that incorporates direct user feedback throughout the development cycle in order to reduce costs and create products and tools that meet user needs.
Two international standards define usability and human-centered (or user-centered) design:
In this section of the Usability BoK
The profession has a deep history, responding to user needs in many domains since the early days of the industrial revolution.
There are a wide range of organizations that support user experience practitioners and researchers, as well as further disciplines that are more generally related to UX skills areas. Organizations are provided, along with links to their web sites if you want more information.
The UPA actively supports practitioner development, whether to hone existing skills, extend into new practice areas, or update knowledge. At the same time, there are other organizations that support UX professional development.