What does cassette mean?

cassette

1. A small flat case containing two reels and a length of magnetic tape that winds between them, often used in audio and video recorders and players and as a medium for storing data in digital form.

2. A lightproof cartridge containing photographic film or plates, used in specially designed cameras.

3. A cartridge for holding and winding typewriter or printer ribbon while in the machine.

Source: Answers.com

In bicycle lingo, cassette refers to the cluster of sprockets, all with different tooth counts, that are found on bicycles with external gears.