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