| Definition -
(Aka noun premodifier)
An optional noun that modifies another noun, i.e. it is optional because it can be removed and the sentence remains well-formed. For example, in the phrase chicken soup the noun adjunct chicken modifies the noun soup.
Oxford English Dictionary - The term's first citation is from 1962:
"Among the nouns is a considerable subclass including, United States, … Hague,.… These are … always preceded by the except in *noun-adjunct position."
(H. A. Gleason in Householder & Saporta Probl. Lexicogr. 93)