Plain English Meaning
A fixed starting line on the aircraft that the manufacturer chooses. Every weight on the airplane (seats, fuel, baggage, passengers) is measured as a distance forward of or behind this line. That distance is what's used to calculate balance.
Full Definition
An imaginary vertical plane or line from which all horizontal distances are measured for weight and balance purposes. The location of the datum is established by the aircraft manufacturer and is shown in the aircraft's Type Certificate Data Sheet and weight and balance documents.
Context Anchor
Seen in weight-and-balance charts, loading examples, and aircraft records that list distances for seats, fuel, baggage, and equipment.
Derivation
From the Latin 'datum,' meaning 'something given.' In aviation, it is literally a 'given' — chosen by the manufacturer and fixed for that aircraft . Knowing this helps why the datum isn't across all aircraft: each manufacturer 'gives' their own.