The following definition of mapping is adapted from the first edition of the Set Book:
Mapping concerns the relationship between controls and their effects in the world. Nearly all artefacts need some kind of mapping between the controls and effects, whether it is a flashlight, car, power plant or cockpit. An example of a good mapping between control and effect is up and down arrows used to represent the up and down movement of the cursor, respectively, on a computer keyboard.
Thus, where there is a clear mapping between the control and its effect, the device should be easier to use. The clarity of the mapping often depends upon labelling, usually in the form of words or icons. A negative example is the Start button on the Windows operating system, which does not actually start anything (instead it reveals a menu). This means it is not clear what happens when the button is pressed, so it has poor mapping.