means The part of a vehicle, rocket, or transmission that exists to be carried — the cargo, warhead, or data that's the actual point of the whole operation.

from A plain English compound of 'pay' + 'load,' coined in the early 20th century from a brutally commercial logic: of everything you haul, only the portion that earns its keep — the freight customers pay for — is the 'paying load.' Everything else (fuel, engine, structure) is overhead. The term hardened into engineering jargon for rockets and missiles, where the payload is the satellite or warhead atop a tower of expensive machinery built solely to deliver it, and later leaked into computing, where a data packet's 'payload' is the real message wrapped in headers and addressing.