For me, usability is the goal of having every user accomplish their objective during a visit. Perhaps it is my background in Car Rental, but I find that the best analogy to Web site usability is found on road signs. And the measure of a good road sign is how many travelers reach their destination quickly.
Take, for example, this road sign.
It provides 3 key pieces of information with minimum use of words.
It could have been more specific and read: "If you would like to drive to Phoenix, you will need to exit by taking the next right." (in a Web site analogy, the "next right" would be a hypertext link)
Surely making such a wordy sign would cause a great many accidents and missed turns. For a Web site it might be analogous to abandonment.
There are other usability practices in this sign as well - like white text on a green background. The white appears in a multitude of weather conditions and reflects headlights. But why the green background? Green means "go" for one. But probably more importantly, it is standardized. All road signs (at least on the east coast of the US) are green. It becomes common and expected.
Suppose instead that this one sign was suddenly changed to a purple background. The person driving by would likely spend some time wondering "Why is that sign purple? Is there something special about this exit? Is purple good or is it bad?"
All of these rules apply to usable Web site designs. Users need to be given directions with clear what/why/where answers. The text needs to be kept to a minimum. The signs should be visible under many conditions. And there should be consistency of colors and fonts to keep parallel choices parallel.
Next time you look at a site and try to judge its usability - pretend you are driving by at 60 miles an hour trying not to crash into someone next to you. Would the site get you to your destination? Or would you have to turn around and go back?