the.com/payload bay

means The dedicated compartment in a vehicle—originally a rocket or aircraft—built to carry the cargo, satellite, or equipment that the mission actually exists to deliver.

from 'Payload' comes from early 20th-century freight and trucking usage, where it simply meant the part of a load that earned money, as opposed to the weight of the vehicle itself. The word later got drafted into aviation and then spaceflight, where 'payload' became whatever the rocket or shuttle was actually built to deliver — satellites, crew modules, scientific instruments — and 'bay' (an old word for a compartment or opening, related to bay windows and horse stalls) supplied the physical space for it. Put together, 'payload bay' entered common use with 20th-century aerospace, most famously as the big cargo hold in the Space Shuttle.