suurus või suuruste kogum, mida kasutatakse viite või alusena muude suuruste arvutamisel, any quantity or set of quantities that may serve as a reference or basis for the calculation of other quantities (ISO 19104*), an imaginary vertical plane from which all horisontal measurements are taken for balance purposes with the aircraft in level flight attitude. The datum is indicated in most Aircraft Specifications or TCDS (Type Certificate Data Sheet).