An idiom is an expression that takes on a figurative meaning when certain words are combined, which is different from the literal definition of the individual words.
Idioms have no defining rules. They are, however, phrases that mean something other than the words that create them. Phrases that are common in everyday language and have figurative meanings that are widely understood are idioms. The literal meanings make no sense. Many idioms are also considered cliches because they are used so frequently. If you are using idioms in a written piece, try to avoid those that are overused unless you feel one is absolutely necessary to the purpose of your words. Consider the following example: