A compound noun is a noun formed by joining two or more words together. There are three forms of compound nouns: open/spaced with a space between words, hyphenated with a hyphen, and closed/solid with no space or hyphen. Compound nouns can be formed by combining a noun with a noun, a verb with a noun, or an adjective with a noun. It is important to understand compound nouns as each one acts as a single linguistic unit.