[J]

Jack – A connecting device having springs which make electrical contact with mating contacts of a plug.

Jitter – (1) A distortion caused by the variation of a signal from its references which can cause data transmission errors, particularly at high speeds. (2) Variations in amplitude, time, frequency or phase.

Julian calendar – Calendar that is used in data processing. The dates are five-digit numbers: the first two digits pertain to the year and the last three to the day of the year (001 through 365 or 366).
jump scrolling – Vertical movement (up or down) of data lines, one line at a time, on a cathode ray tube (CRT). Contrast with smooth scrolling.

Justify – (1) To position characters of a text so that the left or right margins are aligned. (2) To order digits so that the least significant or most significant is always at the left or right margin.