Nouns as modifiers See the glossary definition perform the same functions as adjectives in that they give more information about the headword See the glossary definition (noun) in the noun phrase. The tightest connections between nouns occur in noun compounds where the two words have one well understood meaning, for example "bus station". And sometimes they have become just one word, for example "motorcycle".
But we are concerned with a looser connection between a noun as adjective and its headword. This construction is a very powerful means of packing a lot of information into a few words.
Like adjectives, we can use a series of nouns to provide more information about the headword or about another noun, for example greenhousenoun describing what type of gas gasnoun describing what category of emissions emissionsheadword (noun).
Nouns as adjectives can be classified into various categories such as location: oceannoun describing where the trenches are trenchesheadword, specialization: healthnoun identifying a particular category of professional professionalsheadword, or composition: tomatonoun of composition (the sauce is made of tomatoes) sauceheadword.