Usability is a multi-dimensional quality of a product, and includes the following key attributes:
Learnability:
The ability of the user to discover and understand product capabilities and how to use them.
Utility:
The completeness and appropriateness of the product in achieving end-user goals.
Efficiency:
The ease of use of the product in helping end-users to accomplish their critical tasks.
Memorability:
The extent to which a casual user can remember how a product works and retain proficiency with it.
Error Management:
How errors are prevented, recovered from, and managed to minimize loss and user frustration.
Satisfaction:
The degree to which users enjoy using and interacting with the product.