write-only memory

Noun

 * 1)  A type of computer memory to which data can be written, but from which it cannot be read.
 * 2)  A failed or otherwise inoperative memory device.

Usage notes
The term originated as a joke, as there was thought to be no possible use for memory that cannot be read, and is still often used in this way (often as a euphemistic way of referring to a failed or otherwise nonfunctional memory device). However, it was soon realized that instances of write-only memory do actually occur in many real-life systems (although, in most real-life examples, the memory is only write-only from the perspective of a particular device, and can still be read by other devices, or by the user).