Microcode
A program inside the processor explaining to the silicon what each instruction actually means.
Maurice Wilkes proposed the idea in 1951: instead of building hardwired control logic for every instruction, store a table of simple microinstructions that together carry it out. That made complicated instruction sets manageable and was a precondition for the IBM System/360, where one instruction set could run on machines of entirely different internal construction.
Modern x86 processors break complex instructions into micro-operations scheduled internally, while simple instructions pass straight through. That microcode can be updated after manufacture has become security-critical: the mitigations for Spectre and Meltdown were delivered partly as microcode updates via BIOS and operating systems, with a performance cost attached.