A notch filter (radio hobbyist slang) refers to a rejector circuit inserted into a path where it is necessary to block a signal of a specific frequency (the notch frequency) from entering a radio device. This could be in an antenna line, power supply wires, or other circuits. A notch filter may be used to eliminate intermediate frequency interference or to prevent unwanted signals from one circuit from coupling into another.
A notch filter is a specific type of band-stop filter. A stop filter designed to suppress a single specific frequency is called a narrowband stop filter or notch filter. It is typically implemented as a two-terminal network (most often a parallel resonant circuit) whose impedance sharply increases within a relatively narrow notch frequency range. The notch filter blocks alternating currents within this frequency range and allows currents with frequencies outside it to pass freely.
Related articles: Bandwidth, Radio Signal Spectrum.