According to Nielsen (http://bkaprt.com/jer/18/), usability is a quality attribute defined by five components:
• Learnability: how easy is it for users to accomplish basic tasks the first time they come across the design? • Efficiency: once users have learned the design, how quickly can they perform tasks? • Memorability: when users return to the design after a period of not using it, how easily can they reestablish proficiency? • Errors: how many errors do users make, how severe are these errors, and how easily can they recover from the errors? • Satisfaction: how pleasant is it to use the design?