Usability is one of those words that has a faint jargon-style feeling to it. In pitching the power of eyetracking, card sorts, and participant design, you are wisest to avoid all those terms. These are terms that alienate your clients. As John Rhodes discusses in Selling Usability, focusing on the customers, rather than the testing, will help people understand the end goal of testing.
To get to that goal, you will need to design a test, perform the test, get results, analyze the rests. After all of that, you will then need to make sense of the data. With eye tracking, for example, you will need to help make sense of heat maps.
Visualizations, when interpreted well and correlated with think aloud information, can translate data into meaning. A final report puts everything together creating meaning out of data. In the end, usability could be said to be the study of users and interfaces. But, you could think of it as understanding customers or consumers, and then finding a way to help your clients see what you have come to understand.