An expressed value, not an exact real value. E.g. a car battery has a nominal voltage of 12V, but in reality it is usually somewhere between 10 and 14,5 depending on the situation. read more
In measurement, a nominal value is often a value existing in name only; it is assigned as a convenient designation rather than calculated by data analysis or following usual rounding methods. The use of nominal values can be based on de facto standards or some technical standards. read more
In engineering it should not mean "average", or "approximate", even though some young engineers might use it that way. For example, a 1/2" pipe has a nominal size of 1/2" even though basically nothing about it is anywhere close to .500" "Half-inch" is its name. read more