hyphens in compound nouns
 
A single noun which is made up of two or more words is called a compound noun (e.g., cooking-oil, pickpocket). The rules on whether to put a hyphen, a space, or nothing between the words are inconsistent. Therefore, it is best to use a spellchecker or a dictionary to see what versions of the word are acceptable. There is sometimes a requirement to use a hyphen(s) to eliminate ambiguity.
 

Use a Hyphen to Eliminate Ambiguity

There are no specific rules on forming compound nouns. For example, ink-well can be also be written ink well or inkwell – all are correct spellings. However, you should use a hyphen to eliminate ambiguity. Ambiguity is particularly prevalent when the first word of the pairing is a substance (like water or ink).

Examples:

water-bottle / water bottle
(When the first word is a substance, a hyphen is used to show that the item is not made of that substance.) 

ice-axe / ice axe
(Both are acceptable, but ice-axe makes it clear that the axe is not made of ice.)

paper-clip / paper clip / paperclip
(All 3 are acceptable. However, if the clip were made of paper, then only paper clip could be used.)

Please could you pass me that plastic wire-fastener?
(a fastener made of plastic, i.e., not wire)

There is also some ambiguity when the first word of the pairing ends ing. (This is called a present participle.)

Examples:

changing-room / changing room
(Both are acceptable, but changing-room makes it clear that the room is not
changing.)

laughing-gas / laughing gas
(Both are acceptable, but laughing-gas makes it clear that the gas is not laughing.)
 
NOT ALL HAVE A ONE-WORD VERSION 

Be aware that not all compound nouns have a one-word version. Even though inkwell and paperclip are fine, iceaxe and waterbottle are spelling mistakes. There are no rules governing this – you have to know.
 
 
USE YOUR SPELLCHECKER

It's a good idea to test the one-word version with a spellchecker or a dictionary. You can't test the hyphenated version or multiple-word version with a spellchecker because it will just test the spelling of each word (even with the hyphenated version). In other words, if you check pick-pocket or pick pocket (which should be pickpocket), your spellchecker will not highlight it as an error.
 

See also:

Forming the plurals of compound nouns
What are nouns?
The different types of nouns
Alternatives to hyphens in compound adjectives
Hyphens in compound adjectives
Hyphens in prefixes