Datum

An imaginary vertical plane or line from which all measurements of arms are taken. The datum is established by the manufacturer. Once the datum has been selected, all moment arms and the location of CG range are measured from this point.

Aircraft Weight and Balance Handbook, p. G-2

An imaginary vertical plane or line from which all measurements of arm are taken. The datum is established by the manufacturer. Once the datum has been selected, all moment arms and the location of CG range are measured from this point. D.C. Direct current.

Pilot's Handbook of Aeronautical Knowledge, p. G-9