What is an idiom?
“Idioms are words, phrases, or expressions that are unusual either grammatically, as in ‘Long time, no see!’ or there is a meaning that cannot be derived from the conjoined meanings of its elements, as in ‘It’s raining cats and dogs!’ Every language has idioms, and they are challenging for foreign students to learn. – English-Zone.Com
“A speech form or an expression of a given language that is peculiar to itself grammatically or cannot be understood from the individual meanings of its elements, as in keep tabs on.” (There are other meanings too at Dictionary.com.)
“Idioms are phrases and sentences that do not mean exactly what they say. Even if you know the meaning of every word in the idiom you see or hear, you may not understand the idiom because you don’t understand the culture behind it.” – Brigham Young University site. (I had to insert some words in that to make it make sense! But the site is still good.)
What is the difference between idiom and slang?
“Idiom is yesterday’s slang and slang is tomorrow’s idiom. In other words, idiom is slang that has, through use and over time, become acceptable to use in informal language.” – Lindsay Lyons (NZ — site no longer available).
English Idioms – Cambridge International Dictionary of Idioms and the Cambridge Dictionary of American Idioms online.
Idiomsite: find out why you say what you say (American).
English Idioms & Proverbs at Brigham Young University (American).
ESL Cafe Idiom Pages is American but easy to use.
English-Zone.Com Idioms Page – good, but not all available unless you register.
Australian Slang – What does it all mean?
[Update 18 September 2007.]