A term originally used to describe an attack transient heard when playing certain Hammond organs. Each key mechanism contains nine contacts, one corresponding to a single drawbar, with a voltage-carrying busbar in between. As a key is depressed, all nine contacts under the key close against their respective busbars at slightly different times and “bounce” as they close. The sine waves from the constantly running tone generators momentarily connect at random points in their oscillation, causing an audible pop or click.
Originally, key click was considered to be a design defect and Hammond worked to eliminate (or at least reduce) it. However, jazz and rock players liked the sound and it became part of the Hammond experience.