A "distortion" is the alteration of the original shape (or other characteristic) of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted. In some fields, distortion is desirable, such as electric guitar (where distortion is often induced purposely with the amplifier for an apparently louder sound). The slight distortion of analog tapes and vacuum tubes is considered pleasing in certain situations. The addition of noise or other extraneous signals (hum, interference) is not considered to be distortion, though the effects of distortion are sometimes considered noise.
In telecommunication and signal processing, a noise-free "system" can be characterised by a transfer function, such that the output can be written as a function of the input as
When the transfer function comprises only a perfect gain constant ''A'' and perfect delay ''T''