A compound word is a combination of two or more words that function as a single unit of meaning. There are three types of compounds: Those written as single words, with no hyphenation, are called closed compounds--the word "flowerpot," for example. Hyphenated compounds, such as "merry-go-round" and "well-being," are the second type. Those in the third group, called open compounds, are written as separate words--the nouns "school bus" and "decision making," for example.