A nonode is an electron tube constructed with nine internal electrodes: a cathode, an anode, and seven intermediate grids. These complex tubes were designed for highly specific signal processing tasks, such as FM detection or specialized gating circuits in early television and radio equipment. The multiple grids allowed a single envelope to perform functions that would otherwise require several simpler tubes.
Heard on the job
You rarely see a nonode outside of an antique radio chassis, since those FM detector circuits were quickly replaced by solid-state diodes.