The extent to which specified users can use a product to achieve specific goals with effectiveness, efficiency, and satisfaction in a specified context of use–ISO 9241-11 Guidance on Usability. For example, an eCommerce website should let you purchase items easily.
Learnability: A measure of how quickly a user can learn to use the system.
For example, instructions, etc.
Effectiveness: A measure of how well the design enables its users to do what they want to do on the website with no issues.
For example, browsing and purchasing books, making a phone call, uploading and sharing images, etc.
Efficiency: A measure of how much time and effort it takes to complete the task.
For example, on Amazon, there are 1-click payments, Single Sign-on, etc.
Memorability: A measure of how memorable a design is, that even after a long time, the users can still complete the tasks/goals effectively.
For example, opening the door, using an ATM, etc.
Errors: A measure of how the design prevents users from making errors and enabling users to recover from errors if they have committed one.
For example, typos, wrong ID numbers, and passwords, etc.
Satisfaction: A qualitative measure of how the design appeals to the users in terms of ease of use and overall appeal that ensures that users would come back again.