What Is Usability?
Usability is the measure of the quality of a user's experience when interacting with a product or system — whether a Web site, a software application, mobile technology, or any user-operated device. Usability is a combination of factors that affect the user's experience with the product or system, including:

• Ease of learning: How fast can a user who has never seen the user interface before learn it sufficiently well to accomplish basic tasks?
• Efficiency of Use: Once an experienced user has learned to use the system, how fast can he or she accomplish tasks?
• Memorability: If a user has used the system before, can he or she remember enough to use it effectively the next time or does the user have to start over again learning everything?
• Error Frequency and Severity: How often do users make errors while using the system, how serious are these errors, and how do users recover from these errors?
• Subjective Satisfaction: How much does the user like using the system?

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