A notch filter is a type of band-stop filter, which is a filter that attenuates frequencies within a specific range while passing all other frequencies unaltered. For a notch filter, this range of frequencies is very narrow.
The range of frequencies that a band-stop filter attenuates is called the stopband. The narrow stopband in a notch filter makes the frequency response resemble a deep notch, which gives the filter its name. It also means that notch filters have a high Q factor, which is the ratio of center frequency to bandwidth.
A notch filter can be either active or passive depending on the design. A passive filter is one made of only passive elements, i.e. resistors, capacitors, and inductors. Active filters contain an amplifying element, such as an op amp, which is used in some notch filters.
Notch filters are used to remove a single frequency or a narrow band of frequencies. In audio systems, a notch filter can be used to remove interfering frequencies such as powerline hum. Notch filters can also be used to remove a specific interfering frequency in radio receivers and software-defined radio.
Notch filters can be created from a combination of low pass and high pass filters. See App Note 431: Switched-Capacitor IC Forms Notch Filter for one method of making a notch filter using a Switched-capacitor filter (SCF).