Wiktionary
n. (cx linguistics English) a certain kind of Chinese set phrase originating in Classical Chinese, typically four characters in length
Wikipedia
Chengyu (, pinyin: chéngyǔ, lit. " set phrases") are a type of traditional Chinese idiomatic expression, most of which consist of four characters. Chengyu were widely used in Classical Chinese and are still common in vernacular Chinese writing and in the spoken language today. According to the most stringent definition, there are about 5,000 chengyu in the Chinese language, though some dictionaries list over 20,000.
They are often referred to as Chinese idioms or four-character idioms; however, they are not the only idioms in Chinese.
Usage examples of "chengyu".
The Chengyu Expressway from Chongqing to Chengdu was somewhere close, but at this late hour on a Sunday night, few cars would be traveling.