Difference Between Metaphor and Idiom

This is just a brief guide to the difference between idiom and metaphor, something which shows up very often as a source of confusion.

The Cambridge dictionary offers this definition of idiom:

"a group of words whose meaning considered as a unit is different from the meanings of each word considered separately."

and Cambridge gives us this definition of metaphor:

"an expression that describes a person or object by referring to something that is considered to possess similar characteristics."

Very often, an idiom has no association to metaphor, being simply a phrase that becomes adopted by language as if a single word. These idioms are not readily confused with metaphor, though there are times when an idiom is also a metaphor or metaphor system. A good example is the "carrot and stick".

The "carrot and stick" idiom refers to the use of enticement and punishment to motivate a horse or donkey. The carrot was dangled before the animal as a lure, while the stick was used to reprimand stubbornness. Without knowing the relationship between carrot and stick, the group of words seem out of place in a sentence, which is central to its identification as an idiom.

However, in its common use, this idiom makes metaphorical equations, such as this one:

Iran: West's carrot & stick method failed

http://timesofindia.indiatimes.com/articleshow/2236065.cms

Here, the West is seen as making the following equations:

1. Iran is equated to a donkey, being stubborn and unwilling to change its position.

2. Trade incentives are equated to the carrot, aimed at luring Iran from its current position.

3. U.N. sanctions are equated to the stick, used to force a change in Iran's position.

4. The West is equated to the farmer, and is assumed by this model to own or control Iran.

Extending this further, the West may accuse Iran of "digging in its heels" as a way of protecting its position. For example, since the stick equals sanctions of some kind, say, cutting grain shipments to Iran, Iran may "dig in its heels" by stockpiling current grain supplies in preparation. The donkey of this metaphor system has as many parts as we choose to give it.

The way to spot those times when an idiom behaves as a metaphor is to look for signs of an equation being made, then check to see if the equation can be extended, as you see in the above example. There may even be times when you discover that it never really was an idiom after all. It's only natural - cases of mistaken identity are very common among idiom, metaphor and simile.