What term is used for a common expression that does not make sense if taken literally, such as "on the double"?

The term "idiom" refers to a common expression or phrase whose meaning cannot be understood from the literal meanings of the individual words within it. For example, "on the double" means to do something quickly or with haste, rather than actually referring to doubling something in a mathematical sense. Idioms often reflect cultural nuances and figurative language, making them unique to specific languages or regions. This characteristic distinguishes idioms from other forms of figurative language, such as metaphors or similes, which do involve comparisons but do not necessarily encapsulate a phrase's non-literal meaning in a commonly understood manner.