What Is a “Fast” Camera Lens?

Photography has a lot of confusing terminology, and often there are several ways to describe the same idea. One of the more annoying ones is when lenses are called “fast”. How can a lens have a speed? If you drop two lenses, they’ll both hit the ground (and break) at the same time. It’s an old term, so let’s find out.
by Harry Guinness via How-To Geek