Jack: A type of audio connector originally developed for telephone switchboards, and sometimes called a phone connector. Still in common use for musical and other audio equipment. More info: Jack Connectors
Jackfield: AKA Patch-panel. A board which accommodates connections between multiple sources. More info: Patch Panels
Jib: A revolvable camera mounting arm, which can be attached to a dolly or crane.
JPEG: Joint Photographic Experts Group. A standard for still-image compression. Related info: Using JPEG images on the Internet
Jump Cut: A video transition in which one shot appears to "jump" to another shot with very similar framing. Usually considered undesirable but can be used for dramatic effect. More info: Jump cuts
Jump the Shark: A colloquial term used mainly in the US, referring to a defining point in a TV or movie series that marks the beginning of a terminal decline in quality and/or popularity. It stems from the Happy Days episode in which the Fonz literally jumps a shark while waterskiing.More info: Jump cuts