Usability looks at the quality of the user experience and tries to determine how successfully a user can complete a task and how satisfying a device or interface is to use. Including usability in your workflow is also a very important part of good designing and development.
Areas where user interaction can be improved therefore come under usability. While usability and accessibility (strictly speaking) are different disciplines, there are close connections and crossovers between them.
Following are some definitions of usability picked from some good websites:
"A measure of how easy it is for a user to complete a task. In the context of Web pages this concerns how easy it is for a user to find the information they require from a given Web site."
"The ease with which a system can be learnt or used. A figure of merit or qualitative judgment of ease of use or learning. Some methods of assessing usability may also express usability as a quantitative index."
"The effectiveness, efficiency...