J2 - Wide-area digital transmission scheme used predominantly in Japan that carries data at a rate of 6.312 Mbps.

Jitter - Analog communication line distortion caused by variations of a signal from its reference timing position.

JPEG (Joint Photographic Experts Group) - An ISO Standards group that defines how to compress still pictures.

Juice - Slang for electric current or power.

Jumper - A patch cable or wire used to establish a circuit, often temporarily, for testing or diagnostics; also, the devices, shorting blocks, used to connect adjacent exposed pins on a printed circuit board that control the functionality of the card.

Junction Box - Box inside which low-voltage power cable joints are made.