What is the cargo of a spacecraft called?

The cargo carried by a spacecraft is called its payload.

In spaceflight, payload means the useful object or equipment a launch vehicle carries to its destination. Depending on the mission, it may be a communications satellite, scientific instrument, space telescope, cargo spacecraft, biological experiment, or crewed vehicle. The launch vehicle’s engines, fuel tanks, guidance systems, and structure are generally considered part of the rocket rather than the payload.

Payload is not limited to items delivered into orbit. A spacecraft traveling to the Moon or another planet may carry instruments that collect data, while a cargo vehicle bound for the International Space Station may carry food, water, spare parts, experiments, or fuel. In a crewed mission, the spacecraft or crew capsule can itself be treated as the payload of the rocket that launches it.

A common mix-up is confusing payload with payload capacity. Payload is the carried mission object or mass; payload capacity is the maximum amount a launch system can transport to a specified orbit or destination. Rockets protect many payloads inside an aerodynamic payload fairing during ascent.