Analog to Digital Converter
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.