What is the difference between a phrase, an idiom and an expression? I am looking for the context where one is more appropriate than the rest. I've been using idiom and phrase more or less interchangeably.
This question asks nearly the same thing.
Expression has about the same meaning as phrase, except it is usually used of a phrase which is in common use.
So an idiom is a certain sort of expression, which in turn is a subset of phrase: idiom > expression > phrase.
We have three terms here: expression, idiom and phrase.
An expression is a saying. It may be a colloquial, less formal (or more polite) way of expressing something in less literal terms:
A phrase is a semantically meaningful sequence of words. A phrase is not typically a complete sentence. I consider a phrase to be more like a grammatical construct e.g.
Idiom is more subtle. It can be informal or merely non-literal, and either a phrase or a complete sentence. Here are two examples of idioms that are also phrases:
Here's the subtle part: It requires an idiomatic understanding of language to use these non-obvious forms of speech correctly. Idioms resemble metaphors, but are more general.
An idiom can be a single word, if it's used in a way that isn't its literal or dictionary meaning. For example, "moonlighting" to mean having a second job is an idiom. A phrase is any clump of words ( "that dog over there", "my cousin's wife's sister", "because it was hot", "as fast as I can"), and an expression is a phrase that is an idiom.
So: all expressions are phrases, and all expressions are idioms. But some phrases are not expressions, and some idioms are not phrases or expressions.
Further, "just because you say it doesn't make it a saying": opinions may vary on whether a particular phrase is an expression or not.