A compound noun is a noun made up of two or more words that function together as a single noun. Examples include "toothbrush," "playground," and "password." Some compound nouns are written as one word, while others are written as separate words. They can be written in two ways: open or closed. Open compound nouns are written with a space between the words, and closed compound nouns are written as one word.

There are several types of compound nouns, including:

Noun + Noun compounds: These compound nouns are formed by combining two nouns together, such as "toothbrush" or "playground."

Adjective + Noun compounds: These compound nouns are formed by combining an adjective and a noun, such as "blackboard" or "sweetheart."

Verb + Noun compounds: These compound nouns are formed by combining a verb and a noun, such as "swimsuit" or "sunscreen."

Preposition + Noun compounds: These compound nouns are formed by combining a preposition and a noun, such as "inside" or "underground."

It's also important to note that compound nouns can be written in different ways like : closed, open, hyphenated. Closed compound nouns are written as a single word, open compound nouns are written as two separate words, and hyphenated compound nouns are written with a hyphen between the words.