Jitter is the deviation of some aspect of the pulses in a digital signal. In audio applications, Analog to Digital converters are one of the most critical components that are susceptible to jitter. Using an internal or external clock, an A/D converter "slices" an incoming audio signal into a number of pulses that digitally conforms to the original source. These pulses have to occur and proceed with very strict timing, and any periodic displacement of the signal from its ideal location will cause jitter. Jitter can be caused by the converter having a poor clock, or when source impedance is incorrectly matched at the load. The results are heard as low level distortion or noise, and since high frequencies are more susceptible to jitter, a loss of high end definition. The issue is that once jitter has been introduced into the digital data it is impossible to remove it.
(See A/D Converter, D/A Converter)