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Definition: microcontroller

A single chip that contains the processor (CPU), non-volatile memory (flash memory or ROM) for the program, volatile memory (RAM) for processing the data, a clock and an I/O control unit. Microcontroller units (MCUs) are available in numerous sizes and architectures, and billions of these "computers on a chip" are embedded each year in products from toys to appliances to automobiles. For example, new cars can employ a hundred or more, each one very often assigned the simplest of tasks such as monitoring a single button on the dashboard to trigger an event when the driver presses it (see embedded system and automotive systems). See CPU, RAM, ROM and clock.

Motorola 6801 - One of the First
Introduced in 1978, the 6801 was one of the first semiconductor products to claim the "computer on a chip" moniker. The magnified photos show all 256 bytes of RAM memory with barely six bits revealed at the bottom (400x).

They Don't Get Much Smaller
These 8-bit microcontrollers from Microchip are used in myriad applications and cost less than 50 cents per unit. We're not great technology predictors. In 1949, Popular Mechanics speculated that future computers would only weigh "one and a half tons"!