What defines a "compact" road bike? I've been reading a lot about road bikes and looking at the differences between bikes called "compacts" but I don't see much consistency.

I've read that compact road bikes have different gearing and also a sloping top tube but not necessarily both. I've also seen some bikes with "Compact" in the title but don't appear to have either of those features! Am I missing something?
The word "compact" can be used in two contexts, both separate from each other.

The first is to do with the geometry of the frame. This is basically where the top tube is sloped rather than level. As is often the case, Sheldon has a good description including the whys and wherefores.

The second is to do with the gearing, in particular the number of teeth on the front rings. This is covered in a previous question: Number of gears for a racer. (Its covered on Sheldon too, close to the previous link.)

If you weren't already aware, Sheldon's site is a superb reference for all things bicycle.