An idiom is a word or phrase that has developed a meaning that is different from the one from which the words of the idiom would originally mean? Confused? It's not difficult. Think of phrases such as "raining cats and dogs," "kick the bucket," "straight from the horse's mouth," and "once in a blue moon."
Now, we've never witnessed cats and dogs actually falling from the sky, we're pretty sure that kicking the bucket would hurt, we would argue against pulling anything from a horse's mouth, and we've never seen a blue moon. The thing is that these colorful phrases are idioms; they are phrases that developed over time to have meanings other than what they were originally.
All languages have idioms. They are one of the components of language that make learning a language other than your native tongue difficult. If you told a new English learner that it was raining cats and dogs outside, he or she would likely be very confused... or think you were seeing something.
Idioms are fun. Can you identify each one in this quiz?