Usability is a combination of factors that effect the user s experience with the product.
The degree to which a product enables a user to do his work and achieve his goals. Learnability The measure of how rapidly a user can become productive. A measure of how rapidly an infrequent user can re-learn the product after periods of not using it. Effectiveness The measure of user productivity and how well a user can perform his job. The measure of how quickly a user can perform work and the error rate in doing so.