(Short for "analog to digital converter", often abbreviated to ADC) An electronic circuit that converts an analog signal and converts it into a stream of digital words that represent the input signal. It does this by periodically "sampling" the instantaneous voltage of the analog signal and transforming it into a digital word that represents the voltage. The process of proper conversion is driven by a clock signal such that the conversion are done at precise intervals. Per the mathematics involved, the clock signal that drives the conversion process must be at least twice the frequency of the highest frequency to be converted; this frequency is known as the Nyquist frequency. The input signal must be pre-filtered to ensure that it does not contain any frequencies above the Nyquist frequency; otherwise, a form of distortion known as aliasing will result.
The A/D converter is an essential part of any sampler, and any other audio device that accepts analog signals and then processes them in the digital domain. There are a variety of design types. The opposite of an A/D converter is the D/A converter.