jack - A place to plug in electrical wires or connectors.

jaggies - The jagged edges that appear in graphics on diagonal or curved lines. The stair-step appearance is the result of trying to build a diagonal or curved line out of little squares.

Java {JAH-vah} - A network-oriented programming language invented by Sun Microsystems that is specifically designed for writing programs that can be safely downloaded to your computer through the Internet and immediately run without fear of viruses or other harm to your computer or files. Using small Java programs (called "applets"), Web pages can include functions such as animations, calculators, and other fancy tricks.

JavaScript {JAH-vah skript} - scripting language commonly used to construct Web pages. Programmers use JavaScript to make Web pages more interactive; for example, to display forms and buttons. JavaScript can be used with Java, but is technically a separate language. Java is not required for JavaScript to work correctly.

jewel case - A plastic container for a CD-ROM disc.

joystick - A device consisting of a hand held stick that pivots about one end and transmits its angle in two dimensions to a computer. Joysticks are often used to control games.

JPEG {JAY-peg} [Joint Photographic Experts Group] - Format for storing high-quality color and grayscale photographs in bitmap form; also the group that developed the format. JPEG provides lossy compression by segmenting the picture into small blocks that are divided to get the desired ratio; the process is reversed to decompress the image.

jukebox - A machine that holds and plays CDs, vinyl records, videodisks, tape cartridges, or CD-ROMs. The jukebox automatically picks up disk or tape the user has selected, and moves it to the drive to be played.

jumper - A small, removable plug whose presence or absence is used to determine some aspect of hardware configuration.