Plain English Meaning
A fixed starting line on the airplane that the manufacturer chooses. Every weight on the aircraft is measured by how far it sits from this line, so you can work out where the airplane balances.
Full Definition
An imaginary vertical plane or line, established by the aircraft manufacturer, from which all horizontal distances are measured for weight and balance purposes. Distances measured aft of the datum are positive; distances forward are negative.
Context Anchor
You will see this term in weight and balance charts, loading problems, and aircraft records that show where equipment, fuel, baggage, or passengers are located.
Derivation
From Latin 'datum,' meaning 'something given.' The manufacturer 'gives' a fixed reference point, and all other measurements follow from it. This helps why the location is somewhat arbitrary -- it's chosen by the manufacturer, not dictated by .