Distortion
Definition
Any alteration of an audio signal that adds harmonic content not present in the original. Can be unintentional (clipping, equipment failure) or intentional (saturation, overdrive, harmonic enhancement).
In Simple Terms
Any change to a sound that adds new frequencies not in the original. Sometimes it's bad (clipping), sometimes it's intentional and sounds great (tube warmth, guitar overdrive). Context is everything.