This is an excellent video that explains how Usability fits into User Experience Design, by Wisc-Online.
The effectiveness, efficiency and satisfaction with which specified users achieve specified goals in particular environments.
1. Effectiveness: the accuracy and completeness with which specified users can achieve specified goals in particular environments
2. Efficiency: the resources expended in relation to the accuracy and completeness of goals achieved
3. Satisfaction: the comfort and acceptability of the work system to its users and other people affected by its use
Video length is 3 minutes and 51 second.