A compound noun is a noun made up of two or more words that work together to form one noun. Most compound nouns are made up of one main noun with another word describing it in some way. Other compound nouns might be made up of two words that are not nouns at all on their own.
Blueberry, for example is made up of the noun berry and the adjective blue. A blue berry could be any berry that is blue, but a blueberry is one specific kind of berry.
Cell phone is also made up of the adjective cell and the noun phone. Of course, both words are shortened from cellular and telephone respectively.
Coffee table is made up of two nouns. Both coffee and table refer to very different things on their own, but when put together they refer to a specific type of table.
Undertaker is made using the preposition under and the noun taker.
Other compound nouns such as checkup are not made up of any nouns. Check is a verb, and up is a preposition. However, used together, they make a noun.
Some compound nouns such as father-in-law and editor-in-chief are made up of more than two words.
There are three types of compound nouns. Compound nouns made by putting two words together to make one word, such as blueberry and undertaker, are called closed compound nouns. Compound nouns made of words that remain separate, such as cell phone and coffee table, are called open compound nouns. Compound nouns formed by joining words with hyphens, such as father-in-law and editor-in-chief, are called hyphenated compound nouns.
As frustrating as it is, some compound nouns may be correctly written in more than one form. For example, one person might write cellphone while another writes cell phone. One style guide might recommend website, but another guide might prefer web site. For professional and academic writing, it is important to be consistent and follow the rules and style guides provided by an employer or instructor. Otherwise, when in doubt, it is wise to look up the correct or preferred form in a reliable dictionary.