5 Factors of Usability

Usability is ultimately concerned with the following factors:

Productivity. How effectively can a user perform their job using the product or system?

Learnability. How fast can a novice user learn how to use the user interface sufficiently to accomplish basic tasks?

Error Frequency. How often does the user make errors whilst using the product or system and how serious are these errors?

Memorability. Can a user, who has used the product or system before, remember how to use it effectively next time, or does the user have to learn everything from the beginning?

Satisfaction. How much does the user like using the product or system?