The primary system module was a module in Soong-type androids. This module was the "torso" of the androids. The module was one of the main sections of the androids, along with the positronic cortex unit, the spatial orientation servo, the locomotion subsystems, and the substrate interface.
In 2365, when Commander William T. Riker was reviewing the technical specifications for Lieutenant Commander Data, the module was mentioned in the specs, with its components listed. (TNG: "The Measure Of A Man")