An idiom is a turn of phrase which may seem incomprehensible to someone who is not familiar with the language in which it is spoken. As a general rule, idioms are also very difficult to translate, except in a very roundabout way. Idioms are extremely difficult to learn, and many language learners cite them as one of the more challenging parts of thoroughly understanding language and culture. Because idioms are used so frequently and extensively, several companies make idiom dictionaries which list idioms and their proper usage for language learners.
Were someone to look at the words of an idiom alone, they might have difficulty understanding the meaning of the phrase. For example, many English speakers say that someone “kicked the bucket” when they died. This peculiar use of language is not taken literally, but most English speakers understand it, along with thousands of other idioms. In other instances, an idiom may reference a cultural body of knowledge such as literature, which can be confusing to someone who is not steeped in that culture.
Popular sayings are often idioms, as is the case with “let the cat out of the bag.” In this sense, a cat is not literally being let out of the bag, and the listener understands that the speaker is referring to revealing a secret. The user is often unaware of the roots of the idiom, since it has been so deeply integrated into the speaker's culture.
Many idioms are colorful and curious, and their origins are impossible to trace. It is suspected that William Shakespeare added several hundred to the English language, but countless others are constantly acquired and dropped over time. These figures of speech color a language, making it more alive and fun to use. They are also often an important part of a national culture, which can be very alienating to people who are not from that culture.
Some people also use the term “idiom” to refer to a sort of vernacular speech which is unique to a region or people. Someone might speak, for example, of “the American idiom,” referring to the way in which American speakers use English. While this form of English is not entirely a separate dialect, it has diverged radically from British English, making it sound and feel very different. This usage of the word is a reference to its root, the Greek idiousthai, “to make one's own.”