An ELT Glossary : Compound words

Compound words are words which are considered as a single lexical item, although made up of two words which, elsewhere , could be used separately - ie they are free morphemes.  As the examples below show, the two elements can come from a variety of word classes and may be written as a single word, as two separate words or hyphenated. Sometimes it is a matter of choice, eg road block, road-block and roadblock are all possible. However, others are normally written in one way or another (eg footstep as one word), while in some cases the difference may cause a change in meaning. For example, a black bird (not a compound but an an ordinary adjective plus noun construction) is any type of bird which is black in colour - crows, ravens, jackdaws etc are all black birds - while the compound blackbird refers to one specific type of bird (Turdus merula). Similarly,  a green house is any house which is green in colour, whereas a greenhouse is a construction made of glass in which plants are grown. 

Most compound words in English are nouns or adjectives. Some examples :

a) Compound nouns

footstep, headache, police station, road-block  (noun + noun)
whiteboard, blackbird, greenhouse (adjective + noun)
swimming pool, riding boots (verb + noun)
onlooker, underpass (preposition + noun)

b) Compound adjectives

well-known,  short-sighted (adverb + past participle)
world-famous (noun + adjective)
good-looking (adjective + present participle)
over-ripe (adverb +adjective)

Some compounds may, however, have more than one grammatical function. For example :

He has a full-time job : full-time = adjective
He works full-time : full-time =  adverb

They were plotting to overthrow the government : overthrow = verb
They were plotting the overthrow of the government : overthrow = noun

Related Reading

McCarthy, M. 1990, Vocabulary, OUP

(Don't forget that Amazon often has both new and used copies of the books you want at prices lower than those shown here. It's always worth clicking to find out.)