A compound noun is a noun made up of two or more words. Sometimes, compound nouns are hyphenated, but there are plenty of examples of compound nouns that are not hyphenated. There are also compound nouns that are written as one word. Here are some examples:
Some compound nouns that used to be hyphenated are no longer hyphenated, and some compound nouns that used to be two words became one word. The “rules” change based upon common usage.
Paper-clip is now just paperclip.
And healthcare is now generally considered just one word, but some people still say it should be two words, health care.
“Rules” of correctness change constantly, and rules related to compound nouns change rather quickly. If you’re in doubt about how to write a compound noun, be sure to check a good online dictionary.