Distortion

In telecommunication and signal processing, the term distortion has the following meanings:

In a system or device, any departure of the output signal waveform from that which should result from the input signal waveform's being operated on by the system's specified, i.e, ideal, transfer function.
Distortion may result from many mechanisms. Examples include nonlinearities in the transfer function of an active device, such as a vacuum tube, transistor, or operational amplifier. Distortion may also be caused by a passive component such as a coaxial cable or optical fiber, or by inhomogeneities, reflections, etc., in the propagation path.
In start-stop teletypewriter signaling, the shifting of the significant instants of the signal pulses from their proper positions relative to the beginning of the start pulse.
The magnitude of the distortion is expressed in percent of an ideal unit pulse length.

Source: from Federal Standard 1037C in support of MIL-STD-188

Within the world of guitar music and guitar amplification, distortion is a type of effect that is commonly used with guitars, particularly within rock and heavy metal. There are two main ways to achieve distortion - either from the amplifier (sometimes from the preamplifier) or from a stomp box.

In this context, distortion refers to a clipping or compression of the wave form of an input. In fuzz boxes and solid state distortions, the signal is boosted, and the tops of the waveform clipped off. In vacuum tube distortion, or tube modelling distortion, the top of the wave form is compressed, thus giving a smoother distorted signal, that retains more of the data in the original waveform. This is generally considered more pleasing to the ear, as it is closer to total harmonic distortion. This is commonly referred to as overdrive, as it was originally (and still is) attained by driving the tubes in an amplifier a little harder than they can handle without affecting the signal.