A compound noun is a noun that is made up of two or more words. Most compound nouns in English are formed by nouns modified by other nouns or adjectives.

For example:

The words tooth and paste are each nouns in their own right, but if you join them together they form a new word - toothpaste.

The word black is an adjective and board is a noun, but if you join them together they form a new word - blackboard.

In both these example the first word modifies or describes the second word, telling us what kind of object or person it is, or what its purpose is. And the second part identifies the object or person in question.

Compound nouns can also be formed using the following combinations of words:

Noun + Noun toothpaste, bedroom, water tank, motorcycle, printercartridge, policeman, boyfriend, post office, fruit juice

Noun + Verb haircut, rainfall, train-spotting, car park

Noun + Preposition hanger-on, passer-by, full moon

Adjective + Noun bluebird, greenhouse, software, redhead

Verb + Noun swimming pool, washing machine, driving license, dining room

Verb + Preposition look-out, take-off, drawback, breakdown, cutback, drive-in, drop out, feedback, hangover, flyover, hold-up, make-up, set-back, stand-in, take-away, work-over, check-in

Adjective + Verb dry-cleaning, public speaking

Preposition + Verb input, output, overthrow, upturn, outbreak, outcome, outlay, outlet, inlet

Preposition + Noun underground, onlooker, by-stander