What do you call an adjective that becomes a noun?

There isn't a single, specific term for an adjective that becomes a noun. However, there are several ways to describe this phenomenon:

* Nounification: This is a general term for the process of turning a word of one grammatical class into another.

* Substantivized adjective: This is a common term for adjectives used as nouns, emphasizing their new function as subjects or objects in a sentence.

* Abstract noun: When an adjective refers to a quality or concept, it's often considered an abstract noun. For example, "the good" refers to the general concept of goodness.

* Collective noun: Sometimes, an adjective describes a group of people or things, acting as a collective noun. For example, "the poor" refers to a group of people who are poor.

The best term to use depends on the specific context and the type of noun the adjective becomes.

Here are some examples of adjectives used as nouns:

* The beautiful (referring to beautiful things in general)

* The rich (referring to wealthy people)

* The impossible (referring to things that can't be done)

* The young (referring to young people)

Remember that the meaning of the word often changes when it's used as a noun.