Chinese 話頭, "critical phrase" or "head of a word" indicating the source from which thought emerges (compare with the Greek Logos, which means the same thing). Hua Tuo is known in Korean as hwadu, and in Japanese as wato, and is often used interchangably with koan (kung-an).
"Hua means “talking,” “remark,” or “a sentence”；tou means “ends,” applicable either in the sense of the beginning or the ending of something; hua tou thus means “the ends of a sentence.” For example, "Who is the one who recites the name of Buddha?” is a sentence, the first “end” of which is the single word “Who.” To put one’s mind into this single word “who," and try to find the solution of the original question, is an example of the “Hua Tou exercise.” “Koan,” however, is used in a much wider sense than Hua Tou, referring to the whole situation or event, while Hua Tou simply means the ends or, more specifically, the critical words or point of the question." —Garma C. C. Chang, The Nature of Zen
Hua Tou is an approach to meditation that emerged from Chinese Buddhism in the twelfth century. In this method, one mentally posits an unanswerable question, continually, without engaging the intellect, while maintaining sincere interest in answering the question. Example Hua Tuos include, “Where was I before birth? Where shall I be after death?”, "If you return your bones to your father and your flesh to your mother, where, then, would you be?", "Which is your original face?", and "What is Wu?" or simply, "Wu." There are two Chinese words wu: 1. "not, no, nothing, emptiness." 2. "enlightenment, to awaken to the fact, to understand."
Tieh Shan wrote: [...] "I went to Hsueh Yen and followed his instruction in meditating on the one word Wu. On the fourth night sweat exuded all over my body, and I felt very comfortable and light. I remained in the meditation hall concentrating on my meditation without talking to anyone. After that I saw Miao Kao Feng, who told me to continue meditating on the word 無 Wu without a moment of interruption, day or night. When I got up before dawn the Hua Tou (“the essence of the sentence”) immediately presented itself before me. As soon as I felt a little sleepy I left the seat and descended to the ground. The Hua Tou remained with me even while I was walking, preparing my bed and food, picking up my spoon, or laying down the chopsticks. It was with me all the time in all my activities, day and night. If one can fuse his mind into one whole, continuous piece, he cannot help but attain Enlightenment." -Garma C. C. Chang, The Nature of Zen