Compound nouns are nouns formed from two or more words that act as a single unit in context. Compound nouns can contain a space between the two words, they can be hyphenated, or the two words can be combined into one word. Furthermore, they can be made up of nouns, adjectives, verbs and even prepositions.
A compound noun can feature a space between the words used while still referring to a single concept, such as "tennis shoe" or "bus stop." Both of these examples use two nouns to refer to a single object that is related to both the words used without referring only to their individual meanings.
A hyphenated compound noun uses a hyphen to separate two or more words. An example of a hyphenated compound nouns is the adjective "six-pack."
A compound noun can also be formed by placing two words next to one another with no character between them, creating a new word. "Breakfast" and "sunrise" are both compound nouns that combine a noun with a verb to refer to a specific thing.
An example of a compound noun that uses a preposition as one of its elements is "underworld." In some cases, prepositional phrases can be combined with nouns using hyphens, as in the word "mother-in-law."