The result of passing audio through some electronic device. Since it is impossible to make a perfectly linear device (audio out exactly matches audio in) harmonic distortion is always a byproduct of signal processing. The amount of distortion a particular piece of electronics creates is in most cases kept as low as possible, particularly in reference to "transparent" microphone preamplifiers. However, sometimes the "coloring" is acceptable and indeed favored, and can be taken to extremes.