What is defined as a theoretically exact plane, point, or axis?

A datum is defined as a theoretically exact plane, point, or axis that serves as a reference for measurement or alignment in the geometric dimensioning and tolerancing (GD&T) system. It is the origin from which the location and orientation of features on a part are determined and ensures consistency in manufacturing and inspection processes.

By establishing a datum, designers can specify how certain features of a part relate to one another and the environment in which the part will function. Datums are essential for defining the geometric relationships needed for tolerance zones, thus allowing for controlled variations in the manufacturing process while ensuring that the part will fit and function as intended.

In contrast, other terms such as a feature control frame, dimension line, and reference point serve different purposes in GD&T. A feature control frame communicates the tolerance requirements for features, a dimension line indicates a measure of length or distance, and a reference point is often used in a more general context to denote a location within a drawing, but it does not have the exacting nature or authority of a datum in GD&T applications.