Payload

The payload is the mission-specific equipment that a satellite carries to perform its intended function. For a communications satellite, the payload is the set of transponders that relay signals. For an Earth observation satellite, it's the imaging sensors and cameras. For GPS, it's the atomic clocks and navigation signal generators. The payload drives the satellite's design requirements — its mass, power consumption, data rates, and pointing accuracy all flow from what the payload needs. In launch context, "payload" also refers to the entire spacecraft (or batch of spacecraft) a rocket delivers to orbit.