When describing a periodic function in the frequency domain, the term DC coefficient or DC component refers to 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. The term's use is extended to two dimensional transformations like the Discrete cosine transform used in JPEG.
