Plain English Meaning

A starting line chosen by the manufacturer that the airplane uses as 'zero.' Every weight on board is measured by how far it sits in front of or behind that line, so the airplane's balance can be calculated.

Full Definition

An imaginary vertical plane or line, established by the airplane manufacturer, from which all horizontal distances are measured for weight and balance calculations. Distances aft of the datum are positive (+) and distances forward of the datum are negative (-).

Context Anchor

Seen in weight-and-balance charts and loading calculations that show where seats, fuel, baggage, and equipment are measured from.

Derivation

From Latin datum, meaning 'something given.' In engineering, a datum is a fixed reference point that is simply 'given' — agreed upon — so all measurements can be made from the same place. '' just means it's everything else is referred to.