Sometimes an adjective is used in place of the noun that it describes. Is there a technical term for this?
In the past, there have been noun usages called noun adjunct or attributive noun, or noun premodifier. In grammar, nouns can modify other nouns, acting as adjectives. This "hybrid"—the adjective-working-as-a-noun" is optional in that it is not necessary in order for the sentence to make sense. I.e., it can be removed and not change the sentence's grammar. Examples would be "beef stew" and "rain storm." If a sentence reads, "We couldn't go out to play because of the rain storm," the word "rain" can be removed—like an adjective; it simply provides additional, descriptive information. Without it, the sentence still makes sense; the reader is simply unaware of what kind of storm kept the children inside.
...we can use a noun as an adjective when it precedes a noun that it modifies.
In this case, the "modern" term for such a grammatical device is an adjectival noun; it used to mean the same thing as the noun adjunct, but now the term adjectival noun is defined as an adjective used as a noun.
In the past, this process was called "nominalization." As stated above, it is the practice of using one part of speech as another.
Rules for writing adjectival nouns vary. One source states that with an adjectival noun, it may be written as two separate words, such as "dust bowl" or hyphenated. However, another source states that when nouns are joined, the two words should be hyphenated, as with "eye-opener." But with this second rule, for these examples, removing the first part of the word does not allow for the sentence's meaning to remain intact. For example, if we say, "The presentation about raising money for the team was an eye-opener," we cannot remove the word "eye" and have the sentence still make sense. This may be because technically we are not using the "eye" (noun) specifically as an adjective. Some words may be hyphenated, while others are written as "compound nouns," (adjectival nouns)—one word.
All of the sources I checked pointed out the importance of using a dictionary to double-check.
This is just one example of the flexibility of language, especially the English language. Ironically, it is also one aspect of the English language that makes it so difficult to learn.