A flash ADC, or flash analog-to-digital converter, is a type of converter that quickly transforms an analog signal into a digital signal. It uses a series of comparators to compare the input voltage against reference voltages, allowing it to produce a digital output in a single step. This makes flash ADCs extremely fast, suitable for applications requiring high-speed data processing. However, flash ADCs can be expensive and complex due to the large number of comparators needed, especially for high-resolution conversions. They are commonly used in applications like digital oscilloscopes and high-speed data acquisition systems, where speed is critical.