A microprocessor is a central processing unit, the part of a computer that fetches instructions, performs arithmetic and logic, and controls the rest of the machine, implemented on a single integrated circuit. Before the microprocessor, a CPU was built from many separate components: racks of discrete transistors, then boards of small- and medium-scale integrated circuits wired together. Collapsing all of that onto one chip is the defining idea.
The concept was proven commercially by the Intel 4004 in 1971. The Computer History Museum’s Intel 4004 oral history panel records the team’s own framing of the achievement: the MCS-4 announced in November 1971 is recognized as the first commercial microprocessor, a CPU integrated onto a single MOS silicon-gate die. The architectural insight, captured in Ted Hoff and Stan Mazor’s oral history, was that a single programmable processor could replace a sprawling set of fixed-function logic chips, with the differences between applications expressed in software rather than in custom wiring.
That substitution of software for hardware is why the microprocessor mattered beyond any one product. A general-purpose CPU could be manufactured in volume and then programmed for thousands of different jobs, from calculators to traffic lights to personal computers. The economics of integrated-circuit manufacturing, where the cost of a chip falls as volume rises, meant that a single design serving many markets could become extraordinarily cheap.
The physical consequence was a change in scale. The room-sized mainframes and minicomputers of the 1950s and 1960s gave way, over the following decade, to machines whose processing core fit on a fingernail-sized chip. The same stored-program model described by von Neumann decades earlier now ran on hardware that cost a few dollars rather than a fortune.
The microprocessor is therefore less a single invention than a turning point. It took the abstract architecture of a computer and made it a commodity component. Every personal computer, smartphone, game console, and embedded controller that followed is a descendant of the idea that an entire CPU belongs on one chip.