|optical disk or optical disc
A plastic-coated disk that stores digital data, such as music or text, as tiny pits on its surface. The disc is rotated rapidly, and a laser is reflected off the disc to an optical sensor, which is sampled periodically. If the laser strikes the flat area between pits, the sensor picks up the reflection and registers the digital value 1. If the laser strikes one of the pits, it is deflected from the sensor, and the sample value is a digital 0.