What is "noun adjunct"

n. a noun that modifies another noun and that is optional (that is, it can be removed without affecting the grammar of the sentence).

In grammar, a noun adjunct or attributive noun or noun (pre)modifier is an optional noun that modifies another noun; it is a noun functioning as a pre-modifier in a noun phrase. For example, in the phrase "chicken soup" the noun adjunct "chicken" modifies the noun "soup". It is irrelevant whether the resulting compound noun is spelled in one or two parts. "Field" is a noun adjunct in both "field player" and " fieldhouse".