The definition on the board in this Andertoons comic is mostly correct.
Idioms aren’t limited to phrases, however. Idioms can be anything from a single word to several sentences. In fact, small variations in spelling can be idiomatic. Think of writing in a southern accent, or should I say suhthun acksayent?
However, the point of this post is a heads up. If you’re a native speaker of English, you most likely don’t have much trouble understanding idioms, and often don’t even think about them. Here’s the heads up:
When you write something that might be translated, avoid idioms!
You want your readers to understand what you mean without having to puzzle over what you wrote. Idioms don’t lend themselves to effective translation in expository writing, so avoid them.
Here, I’ll give you an example, a German idiom. What does it mean in English? (I’ll translate it literally for you)
That’s like a Bohemian village!
How would you say that in English?
Well, I’m going to almost give away the answer because I found a comic with the answer in it. Which one is it?