Nominal
Nominal scales are the lowest scales of measurement. Numbers are assigned to categories as "names". Which number is assigned to which category is completely arbitrary. Therefore, the only number property of the nominal scale of measurement is identity. The number gives us the identity of the category assigned. The only mathematical operation we can perform with nominal data is to count.

Classifying people according to gender is a common application of a nominal scale.
In the example below, the number "1" is assigned to "male" and the number "2" is assigned to "female". We can just as easily assign the number "1" to "female" and "2" to male. The purpose of the number is merely to name the characteristic or give it "identity".

As we can see from the graphs, changing the number assigned to "male" and "female" does not have any impact on the data -- we still have the same number of men and women in the data set.

Additional examples for everyday nominal scales are zip codes, area of country.