Well actually, it's called "cargo" when it's in a ship because, by definition, cargo is quite literally goods or merchandise being transported by a ship. Despite looking like a compound word of "car" and "go" and being pronounced the same as "car go", it has nothing to do with cars. It's called a "shipment" when it's in a plane because it comes from the root word "ship", which originally referred to transportation by sea then over time gradually went up to meaning general transportation.
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