A band-stop filter, also known as a band-reject or notch filter, is an electronic or electrical filter designed to attenuate or block a specific range of frequencies while allowing all other frequencies to pass through. It is the opposite of a band-pass filter, which allows a specific range of frequencies to pass while attenuating others.

Band-stop filters are used to eliminate or reduce unwanted interference or noise in a signal. They are often employed in various applications, including audio processing, telecommunications, and electronics. For example, in audio equipment, a band-stop filter can be used to remove a specific noise frequency, such as a 60 Hz hum in the power supply, while preserving the desired audio signal.