IIR stands for Infinite Impulse Response, a type of digital filter used in signal processing. Unlike finite impulse response filters, IIR filters can produce an output that continues indefinitely based on past inputs and outputs. This characteristic allows IIR filters to achieve a desired frequency response with fewer coefficients, making them efficient for real-time applications. IIR filters are commonly used in various applications, including audio processing, telecommunications, and control systems. They can be designed using different methods, such as the Butterworth, Chebyshev, or Bessel approaches, each offering unique advantages in terms of stability and frequency response characteristics.