Four-character idioms, or chéngyǔ are a set of traditional idiomatic expressions, most of which consists of four Chinese characters. Chengyu were widely used in Classical Chinese and are still common in Vernacular Chinese writing and Spoken Chinese today. According to the most stringent definition, there are about 5,000 chengyu in the Chinese language, though some dictionaries list over 20,000.

Chengyu are mostly derived from ancient literature. The meaning of a chengyu usually surpasses the sum of the meanings carried by the four characters, as chengyu are often intimately linked with the myth, story or historical fact from which they were derived. As such, chengyu do not follow the usual grammatical structure and syntax of the modern Chinese spoken language, and are instead highly compact and synthetic.

Chengyu in isolation are often unintelligible to modern Chinese, and when students in China learn chengyu in school as part of the Classical curriculum, they also need to study the context from which the chengyu was born. Often the four characters reflect the moral behind the story rather than the story itself. For example, the phrase "破釜沉舟" literally means "break the woks and sink the boats." It was based on a historical account where General Xiang Yu ordered his troop to destroy all cooking utensils and boats after crossing a river into the enemy's territory. He won the battle because of this "no-retreat" policy. Similar phrases known in the West as "burning bridges" or "Crossing the Rubicon". This particular idiom cannot be used in a losing scenario because the story behind it does not describe a failure.

However, that is not to say that all chengyu are born of an oft-told fable; indeed, chengyu which are free of metaphorical nuances pervade amidst the otherwise contextually-driven aspect of vernacular Chinese. An example of this is 言而无信 (literally "speaks yet (is) without trust"), which refers to an individual who cannot be trusted despite what he says, or essentially a deceitful person. The idiom itself is not derived from a specific occurrence from which a moral may be explicitly drawn; instead, it is succinct in its original meaning and would likely be intelligible to an individual learned in formal written Chinese. Note that the only classical-vernacular discrepancy present in this chengyu lies in the fact that the character 言 is no longer used as a verb in modern Chinese.