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.
Plain English
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.
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 explain why the location is somewhat arbitrary -- it's chosen by the manufacturer, not dictated by physics.
Why Pilots Care
A consistent datum reference produces accurate center of gravity results that directly affect aircraft stability and safe flight.
Analogy
It is like the zero mark on a ruler. The ruler can measure many distances, but every measurement depends on knowing where zero starts.
Intuition Check
Do not think of reference here as a book source or a suggestion. In weight and balance, the datum reference is the zero measuring line for the airplane.
Example Sentence 1
The pilot measured each station's arm from the datum reference before computing the loaded center of gravity.
Example Sentence 2
Before loading, the crew verified the location of the datum reference on the aircraft's weight and balance sheet.