Computer system having two or more processing units (Multiple Processors) each sharing main memory and peripherals, in order to simultaneously process programs.

Sometimes the term Multiprocessor is confused with the term Multiprocessing

Multiprocessing

Multiprocessing is the use of two or more central processing units within a single computer system. The term also refers to the ability of a system to support more than one processor and/or the ability to allocate tasks between them...

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While Multiprocessing is a type of "processing" in which two or more processors work together to "process more than one program simultaneously", the term Multiprocessor is referred to the hardware architecture that allows multiprocessing.