What is the difference between an idiom, a cliché and a proverb?
All three are fixed expressions, and they differ in what they do rather than in how set they are.
An idiom is a phrase whose meaning cannot be worked out from its parts. This is the defining feature — non-compositionality.
Kick the bucket, spill the beans, under the weather, bite the bullet, the penny dropped. A competent speaker who knows every individual word still cannot derive the meaning, which is why idioms are among the hardest things in language learning and why machine translation historically mangled them.
Idioms are neutral — using one is not a criticism. English contains thousands and they are unavoidable.
A cliché is an expression that has been used so much it has lost its force. The defining feature is overuse, not opacity.
At the end of the day, think outside the box, a perfect storm, moving forward, low-hanging fruit. Many began as vivid images and were worn smooth by repetition.
The term is inherently critical — calling something a cliché is a judgement. Note that clichés are frequently perfectly clear, which is partly why they persist: they are efficient, immediately understood, and require no effort from writer or reader. The objection is that they signal the writer did not think, and they slide past the reader without registering.
A proverb is a short traditional saying expressing a piece of received wisdom or advice.
Too many cooks spoil the broth, a stitch in time saves nine, don't count your chickens. The defining feature is that it makes a claim about how the world works and functions as guidance.
Proverbs frequently contradict each other — look before you leap against he who hesitates is lost — which suggests they justify decisions after the fact as much as inform them.
The categories overlap. An idiom can become a cliché through overuse; a proverb can be a cliché.
Related terms: a metaphor compares implicitly, a euphemism softens, and a dead metaphor is one nobody notices any more.