Usability is 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.
Jakob Nielsen defined 5 core components of usability: (from Usability Engineering, 1994)
- Ten Usability Heuristics for User Interface Design (Nielsen Norman Group)
- User-Centered Design Basics (usability.gov)
- Ethical Design: The Practical Getting-Started Guide (Trine Falbe)