Abbreviation of "Compact Disc Read Only Memory". It is a disk on which informations converted into bits have been written in such a way that they can't be modified.

          Its size is about 5 inch diameter. The bits representing 0s and 1s are burnt with a laser at the surface of the disk. Like for old musical record disks there is a very thin spiral track. The thickness of that track is half a micron (1 micron is 1 millionth of a meter) and its total length is 5 km! The burning process creates bumps corresponding to bits. Also simply called a CD.

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