In general the expression "off the shelf" refers to all those items which are available on the market and can be freely purchased and plugged in a system.
Off the shelf processor stands for commercial CPUs opposite to the concept of dedicated ASIC or ASIP (Application Specific Instruction-set Processors) which are custom processors with a reduced set of instructions designed to implement a limited and specific group of functions with higher performances than commercial components (e.g. a cryptographic accelerator chip is an example of ASIP).
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Mowgli