idiom

Group of words with a meaning of its own that is different from the meanings of each individual word in the group (for example, ‘It's raining cats and dogs’ means ‘It's raining heavily’ and kick the bucket means ‘to die’); also a style of expression in writing, speech, or music that is typical of a particular period, place, or person (for example, ‘a piece of music composed in the modern idiom’).

Many idioms are used only in spoken and informal language, for example dropping like flies (when many people are becoming ill or dying from the same illness). Others, such as the salt of the earth (people having praiseworthy qualities), are acceptable in more formal contexts.