Usability is a nebulous word that we hear a lot about in my industry. I'm not sure what it is, but I know what it looks like - that kind of thing, you know? It's something we're supposed to make more of, though, that's for sure. In so far as we can be objective about it, it's sort of a utilitarian view (http://en.wikipedia.org/wiki/Utilitarianism) of devices and software and the things that we interact with every day.
When I was at Web2.0 San Francisco last spring I heard a good definition of usability. John Maeda (http://www.maedastudio.com/index.php) said that usability is a Forever Stamp. I like that. A Forever Stamp increases usability by getting more things out of the "user's" way. When someone wants to mail something they want to mail something, not check to see if they have to attach a stamp and a half or paste a quarter on their envelope. They don't have to worry the whole time the thing is in the mail. They just put on the stamp and send it. It's goal oriented; it doesn't expose more of the inner workings of the system (e.g. stamp price changes) than it needs to.
Here's another take. Usability is a clementine. Who doesn't like oranges? Oranges taste great. At least the juicy part does. The pulpy stuff that sticks to the orange as you peel it isn't that great. Oh, and peeling is a pain. They even make special tools for it. And by the time you get done you're up to your elbows in sticky orange juice and half of the orange is crushed.
Clementines are not that way. There's nothing left behind from the skin. It's small enough that you can just zip it open and eat it. If you're really hungry, have two. Peeling is super easy too. I routinely peel one open one-handed as I drive to work.
Oranges were designed by a big corporation. They wanted to make them big, so they look like more of a deal. They wanted to seal in the good stuff so they really stuck that skin on there. They wanted to put seeds in there so there could be more oranges all over the land. Oranges are there for oranges' sake.
Not clementines. They're just for eating.