Analog-to-Digital Converter (ADC)
Analog-to-digital converters take a voltage range and divide that range into chunks. It assigns each chunk a particular number on the range (depending on the resolution). When the ADC measures a voltage, it assigns a number to that voltage, so you have a digital record of that analog signal.
Analog-to-digital converters (ADCs) translate analog signals to digital numbers.
The AVR Microcontroller and Embedded Systems: using Assembly and C by Muhammad Ali Mazidi, Sarmad Naimi, & Sepehr Naimi
In electronics, an analog-to-digital converter (ADC, A/D, or A-to-D) is a system that converts an analog signal, such as a sound picked up by a microphone or light entering a digital camera, into a digital signal. An ADC may also provide an isolated measurement such as an electronic device that converts an input analog voltage or current to a digital number representing the magnitude of the voltage or current. Typically the digital output is a two's complement binary number that is proportional to the input, but there are other possibilities.
A digital-to-analog converter (DAC) performs the reverse function; it converts a digital signal into an analog signal.