Geometric Dimensioning and Tolerancing is a system for defining and communicating engineering tolerances. It  uses a symbolic language on engineering drawings and computer-generated three-dimensional solid models  that explicitly describe nominal geometry and its allowable variation. It tells the manufacturing staff and machines what degree of accuracy and precision is needed on each  controlled feature of the part.

ISO defines GD&T as “geometrical product specifications (GPS)—Geometrical tolerancing—Tolerancing of form,  orientation, location and run-out.” In short, “geometrical product specifications” refer to the shape, size, and  positional relationship of a product, while “tolerance” means the allowable error. “Geometric tolerance” is  characterized by a definition that includes the allowable errors for the form and position in addition to size.