Nominal data is categorical data which values/observations can be assigned a code in the form of a number where the numbers are simply labels. You can count, but not order or measure nominal data.
In the studio, the mining type of textual data is set to nominal. For example, a column called WEATHER with the values: sun, cloud and rain is nominal.
And a column called POSTAL_CODE that has the values 52200 and 75014 is nominal as well in spite of the numerical values. Such data is of nominal type because it identifies a postal code in France. Computing mathematical quantities such as the average on such data is non sense. In such a case, you should set the data mining type of the column to Nominal, because there is currently no way in the studio to automatically guess the correct type of data.
The same is true for primary or foreign-key data. Keys are most of the time represented by numerical data, but their data mining type is Nominal.