A kōan (pronounced /ˈkoʊ.ɑːn/; Chinese: 公案; pinyin: gōng-àn; Korean: gong'an; Vietnamese: công án) is a fundamental part of the history and lore of Zen Buddhism. It consists of a story, dialogue, question, or statement, the meaning of which cannot be understood by rational thinking but may be intuited. One widely known kōan is "Two hands clap and there is a sound; what is the sound of one hand?" (oral tradition attributed to Hakuin Ekaku, 1686–1769, considered a reviver of the kōan tradition in Japan). The word koan, the name by which practice is known to the West, comes from the Japanese pronunciation of the Chinese characters.
http://en.wikipedia.org/wiki/K%C5%8Dan