Other forms: idioms
An idiom is an expression particular to a specific language or culture. An idiom doesn't make literal sense: Telling someone to "break a leg" isn't wishing them harm; it's wishing them good luck!
Idiom comes from the Greek idios, meaning "one's own," "distinctive," or "private." Originally, an idiom was not limited to language; it was any peculiarity or characteristic of an individual person, group, or language. In early linguistics, it referred to any language-specific expressions, even those that were perfectly literal. Over time, the meaning narrowed, and now an idiom is any fixed expression whose meaning cannot be deduced literally, like kick the bucket, meaning "to die." If you're studying a foreign language, idioms are some of the hardest phrases to translate.