Datums are ideal representations of a partís surface, axis or centerplane. The datum is used as one of the axes of the x-y-z coordinate system. Features of a part are located and oriented with respect to the identified datums.
Even though a datum is a representation of a real surface, axis or centerplane, it is assumed to be perfectly flat and/or straight.
The surface, axis or centerplane used to represent the datum is called a datum feature. Datum features are identified on a drawing using a datum feature symbol.
The figure shown below illustrates the use of the datum feature symbol. The applied GD&T tolerance is controlling the perpendicularity of the left surface with respect to datum feature A (the bottom surface).
ASME Y14.5M Definitions
Datum: A theoretically exact point, axis or plane derived from the true geometric counterpart of a specified datum feature. A datum is the origin from which the location or geometric characteristics of features of a part are established.
Datum Feature: An actual feature of a part that is used to establish a datum.