A system to prevent one process corrupting the memory (or other resources) of any other, including the operating system. Memory protection usually relies on a combination of hardware (a memory management unit) and software to allocate memory to processes and handle exceptions.
The effectiveness of memory protection varies from one operating system to another. In most versions of Unix it is almost impossible to corrupt another process' memory, except in some archaic implementations and Lunix (not Linux!). Under Microsoft Windows (version? hardware?) any 16 bit application(?) can circumvent the memory protection, often leading to one or more GPFs. Currently (April 1996) neither Microsoft Windows 3.1, Windows 95, nor Mac OS offer memory protection. Windows NT has it, and Mac OS System 8 will offer a form of memory protection.
[MS DOS EMM386 relevant?]
The basic unit of memory protection is the physical user, composed of one or more logical users engaged in a common cause.