Plain English Meaning
A fixed starting line picked by the aircraft maker. To work out the aircraft's balance, you measure how far forward or behind this line every weight sits.
Full Definition
An imaginary vertical line, established by the aircraft manufacturer, from which all horizontal distances are measured for weight and balance calculations. The location of every item of weight in the aircraft — seats, fuel, baggage, passengers — is given as an arm (in inches) measured from this line.
Context Anchor
Seen in weight and balance sections of the aircraft manual, loading charts, and balance calculation examples.
Derivation
Datum' comes from Latin meaning 'something given' — in measurement, it means a fixed point everyone agrees to start from. The 'reference' part just confirms it's the line all other measurements refer to.