Parasitic mass is a rocket, weapons and Personal Rapid Transit (PRT or PodCars) transportation term used to differentiate between the mass of the payload and the moving mass of all other components of the system. It is the mass to which force must be applied to accelerate that is not payload.
A typical engineering objective is to drive the parasitic mass towards zero. Efficiency gains are achieved as the parasitic mass is reduced.
References
Parasitic mass Wikipedia(Text) CC BY-SA