One of the best definitions of usability is found in a listing of 5 components posted by Jakob Neilsen on his Alertbox website in 2010:
Usability is defined by 5 quality components:
• Learnability: How easy is it for users to accomplish basic tasks the first time they encounter 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?
Note that many of these factors do not exist in a vacuum. Consistency of UI elements supports transference of knowledge from one system to the next.
Best Practice: Re-use interface elements to increase transferability of knowledge between systems. Integrate end-user testing within the development process. Usability testing and focus groups are common research methods.