Jack: An electrical connector found on tape recorders, patch panels, VCR's, etc.
Jitter: 1.) A tendency toward lack of synchronization of the TV picture. It may refer to individual lines in the picture or to the entire field of view.
2.) In data networking, jitter is slight variability (drifting) of the incoming data packets in a central clocked system.
JPEG: 1.) Joint Photographic Experts Group
2.) A ISO/IC sanctioned standard (1991) for compression of full color photographic quality still images