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