When describing a periodic function in the frequency domain, the DC bias, DC component, DC offset, or DC coefficient is the mean value of the waveform (possibly scaled according to the norm of the corresponding basis function of the frequency analysis filter bank). The name comes from the middle 20th century design of electrical line codes for use with transmission channels unable to transmit a DC voltage or current. In such usage, this coefficient represents the useless DC, whilst the coefficients representing various other frequencies are analogous to superimposed AC voltages or currents, hence called AC components or AC coefficients.
A waveform with a zero DC component (that is, the average D.C. component should be 0) is known as a DC-balanced waveform. DC-balanced waveforms are useful in communications systems, because they can be used on AC-coupled electrical connections to avoid voltage imbalance problems between connected systems or components. To be specific, bit errors can occur when a (relatively) long series of 1's create a DC level that charges the capacitor of the high-pass filter used as the AC coupler, bringing the signal input down incorrectly to a 0-level. In order to avoid these kinds of bit errors, most line codes are designed to produce DC-balanced waveforms. The most common classes of DC balanced line codes are constant-weight codes and paired-disparity codes.