The distinction between real value and nominal value occurs in many fields. From a philosophical viewpoint, nominal value represents an accepted condition which is a goal or an approximation as opposed to the real value, which always is actually present. Often a "nominal" value is a de facto standard rather than a typical or average measurement.
When measurement is involved, the real value often has the characteristics of an irrational number. In realworld measuring situations, improving the measurement technique will eventually begin yielding unpredictable least significant digits. For example, a 1 inch long gauge block will measure to be exactly 1 inch long until the measuring techniques reach a certain degree of precision. As techniques improve beyond this threshold, it will become clear that 1 inch is not the real value of the gage block length, but some other number that is always out of reach.
Only counting can yield an accurate and reproducible real value; see metrology for an explanation.
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In various subfields of engineering, a nominal value is one for which the "name" for the value is close to, but not the same as, the actual value. Some examples:
Other cases involve diameter, speed, and volume.
Sometimes the word "nominal" is also used in engineering contexts as a synonym for "normal" or "expected"; for example, The rotor resistances on all the other operating wheels are nominal.^{[1]}
In economics, nominal values are the face value of currency over long periods of time (years), whereas real values have been corrected for inflation.
