Short for Infinite Impulse Response Filter. An IIR is a type of digital process that looks back in time an infinite number of previous samples (theoretically) in order to generate a result, as opposed to an FIR Filter where a fixed number of past samples are considered. An example would be a function where an outbound sample is equal to an average of ALL of the previous inbound samples, but with more weighting on the most previous inbound samples. An IIR filter is meant to mimic the way that analog components such as capacitors work, in that any information fed into them is dissipated over an infinite amount of time. Because they closely mimic the components used in analog filters, IIR filters are more often used in digital EQ‘s and other areas where digital processes are meant to mirror analog components.