A unit load device (ULD) is a container used to load luggage, freight, and mail on wide-body aircraft and specific narrow-body aircraft. It allows preloading of cargo, confidence the containerised load will fit in the aircraft and efficient planning of aircraft weight and balance and reduced labour and time in loading aircraft holds compared with 'bulk-loading' single items of cargo or luggage by hand. Each ULD has its own packing list or manifest so that its contents can be tracked. A loaded aircraft cargo pallet secured with a cargo net also forms a ULD, but its load must be gauged for size in addition to being weighed to ensure aircraft door and hold clearances.

In transportation, freight refers to goods conveyed by land, water or air, while cargo refers specifically to freight when conveyed via water or air. In economics, freight refers to goods transported at a freight rate for commercial gain. The term cargo is also used in case of goods in the cold-chain, because the perishable inventory is always in transit towards a final end-use, even when it is held in cold storage or other similar climate-controlled facilities, including warehouses.