Band rejection is a phenomenon in wave

Wave

In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...

form signals, where a certain frequency

Frequency

Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 or range of frequencies are lost or removed from a source signal.

The term band rejection, when used in electronic signal processing

Signal processing

Signal processing is an area of systems engineering, electrical engineering and applied mathematics that deals with operations on or analysis of signals, in either discrete or continuous time...

, refers to the deliberate removal of a known frequency range - for instance, to compensate for a known source of interference (such as noise from mains (household) electricity

Mains electricity

Mains is the general-purpose alternating current electric power supply. In the US, electric power is referred to by several names including household power, household electricity, powerline, domestic power, wall power, line power, AC power, city power, street power, and grid power...

). A specific frequency is removed using a notch filter.

In most other senses, band rejection is the unintentional loss of signal caused by imperfections in the recording, storage or reproduction of a waveform.