Peruse just a few of the idioms on this or other sites, and you may notice that many idioms don’t look much different than what most of us call slang. Why not call a spade a spade? Is there a difference between an idiom and slang?
In fact, there is not.
Although there is not one universally accepted definition of idiom, a fairly standard definition is “two or more words used together as a unit that has a special meaning not derived from the meanings of the words separately.”
The only difference between slang and any other commonly used parts of the language is that slang is not yet (and may never be) commonly used by all speakers of the language. Here is how the Oxford English Dictionary defines it: “A type of language consisting of words and phrases that are regarded as very informal, are more common in speech than writing, and are typically restricted to a particular context or group of people.”
Once a slang term or phrase is widely accepted into the culture and begins to be used by everyone, rather than by certain groups, it is no longer slang.
Many idioms are used in formal books, and many informal idioms are used in everyday conversation. However, some idioms are slang phrases. These may be considered rude, annoying, uncouth, etc. but they are still idioms.
Why should it be an idiom if I say, it’s raining cats and dogs but slang if I say that’s such a cop-out? As common as the idiom “cop-out” is, you won’t hear people of all ages and walks of life saying it, but anyone might say “it’s raining cats and dogs” to refer to a heavy downpour. Both are idioms, regardless.
For that matter, a “catchphrase” may well be an idiom. We say “read my lips” to mean listen to what I’m saying very closely. But, it’s also a catchphrase, often used mockingly and associated with President George H.W. Bush, who said it during his campaign, “Read my lips; no new taxes.” As I point out on the front page of this site, some idioms are also considered aphorisms.
Idioms are used like words. Therefore, some are heard so much they become cliches. For example, I used a cliche above, call a spade a spade. This phrase is often considered hackneyed, perhaps because it has been used in so many dime detective novels. It fits the definition of an idiom quite well, though.
In other words, all these categories can overlap. While not all slang phrases are idioms, not all idioms are different than slang.