Abbreviation for Analog to Digital Converter.

These devices are commonly placed on input/sensor lines in microcontrollers to allow continuously varying analog signals to be converted into binary signals.

For example, an 8 bit ADC would convert the power levels to one of 256 numbers. This can then be easily interpreted by a program.

This can go an opposite way by using DACs.

A classic Arduino like the Uno has 6 exposed analog pins using internal ADCs allowing you to connect analog sensors.

It is using these, that the Lego RCX is able to have up to three light-sensors or similar devices connected. In the case of the RCX they have much better resolution than 8 bits, allowing for a more sensitive or greater range of variance.