A nominal adjective is a word that normally functions as an adjective but is grammatically functioning as a noun in a sentence. A nominal adjective looks just like a regular adjective, but it is actually a noun by function.
The British once ruled a vast empire.
Normally British would be used as an adjective to describe nouns in constructions like British tea or British people. However, in this example, British stands alone to refer to the British people all on its own. The same idea applies to many other nationalities as well.
The Japanese are an innovative people.
The Swiss are highly committed to neutrality and security.
Nominal adjectives are not exclusive to the realms of nationality and proper adjectives. Many common adjectives may be nominalized.
There were lots of shirts in all different colors, but Sarah only wanted the purple.
Normally we would use a color like purple as either an adjective to describe a noun’s color or as a noun to refer to the color itself. In this case, the context suggests that purple is meant to be an adjective and implies something like “the purple one” or “the purple shirt.”
I think the worst is over.
As with colors, adjectives like worst are usually used to describe something else. In a way, as we saw with colors, using a nominalized adjective in this way implies more, such as “the worst part” or “the worst time.” Constructions like the one in this example are common with other adjectives of comparison as well.
Protect the weak and innocent.
As in the examples above, the nominal adjectives in context make us understand something like “the weak people and the innocent people.”
Here are a few more examples of nominal adjectives in action to give you some more ideas:
Given a choice between two teammates, Steve picked the smarter over the stronger.
It is important to be considerate of the disabled.
Between going to the party and staying home alone, John gladly chose the latter.
Some parents tend to baby their youngest while overburdening their eldest.