Tolerancing is the art and science of using the largest tolerance possible that will still allow a part to function.
Where only a tolerance of size is specified, the limits of size of an individual feature prescribe the extent to which variations in its geometric form, as well as size, are allowed.
If a feature is dimensioned only with a traditional tolerance, then that tolerance governs all of the tolerances for form. You only specify geomtric dimensions and tolerances to supplement the tradiational size/location tolerance.
A starting place for a dimension such as a perfect plane, a center line, or a point.
Datums are perfect, ideal constructs
In reality "perfection" of a datum can mean that it is at least 10 times more accurate than the accuracy of the part being measured.
Used to specify the geometric tolerances. It is divided into two or more zones.