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.

Technically...

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...we can use a noun as an adjective when it precedes a
noun that it modifies.

gt;

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.

Additional
source:

http://www.grammarbook.com/punctuation/hyphens.asp