Compound (linguistics)
Compound words are words that add together to make a new word, like the word side and walk, to make sidewalk. These words often develop over time. They can start as two words joined with a hyphen, and then become a compound word. There are three types of compound words; closed, hyphenated and open. A closed compound word is joined into one word like keyboard or makeup. A hyphenated word is two words joined with a hyphen like mother-in-law. Open compound words are words which are used together like police station, or fire engine.