In the Sinosphere, the word 無, realized in Japanese and Korean as mu and in Standard Chinese as wu,^{[lower-alpha 1]} meaning 'to lack' or 'without', is a key term in the vocabulary of various East Asian philosophical and religious traditions, such as Buddhism and Daoism.

The Old Chinese *ma (無) is cognate with the Proto-Tibeto-Burman *ma, meaning 'not'. This reconstructed root is widely represented in Tibeto-Burman languages; for instance, ma means 'not' in both Tibetan and Burmese.^[1]

The Standard Chinese pronunciation of wú (無; 'not', 'nothing') historically derives from the c. 7th century Middle Chinese mju, the c. 3rd century Late Han Chinese muɑ, and the reconstructed c. 6th century BCE Old Chinese *ma.^[2]

Other varieties of Chinese have differing pronunciations of Chinese: 無. Compare Cantonese mou4; and Southern Min IPA: [bo˧˥] (Quanzhou) and IPA: [bə˧˥] (Zhangzhou).

The common Chinese word wú (無) was adopted in the Sino-Japanese, Sino-Korean, and Sino-Vietnamese vocabularies. The Japanese kanji 無 has on'yomi readings of mu or bu, and a kun'yomi (Japanese reading) of na. It is a fourth grade kanji.^[3] The Korean hanja 無 is read mu (in Revised, McCune–Reischauer, and Yale romanization systems). The Vietnamese Hán-Việt pronunciation is vô or mô.

Some English translation equivalents of wu or mu are:

• "no", "not", "nothing", or "without"^[4]
• "nothing", "not", "nothingness", "un-", "is not", "has not", "not any"^[5]
  1. Pure awareness, prior to experience or knowledge. This meaning is used especially by the Chan school of Buddhism.
  2. A negative.
  3. Caused to be nonexistent.
  4. Impossible; lacking reason or cause.
  5. Nonexistence; nonbeing; not having; a lack of, without.
  6. The "original nonbeing" from which being is produced in the Tao Te Ching.^[6]

In modern Chinese, Japanese and Korean it is commonly used in combination words as a negative prefix to indicate the absence of something (no ..., without ..., un- prefix), e.g., Chinese: 无-线; pinyin: wú-xiàn/mu-sen (無-線)/mu-seon (무-선) for "wireless".^[7] In Classical Chinese, it is an impersonal existential verb meaning "not have".^[8]

The same character is also used in Classical Chinese as a prohibitive particle, though in this case it is more properly written Chinese: 毋; pinyin: wú.^[9]

In traditional Chinese character classification, the uncommon class of phonetic loan characters involved borrowing the character for one word to write another near-homophone. For instance, the character 其 originally depicted a winnowing basket (ji), and scribes used it as a graphic loan for qi (其, "his; her; its"), which resulted in a new character ji (箕) (clarified with the bamboo radical ⺮) to specify the basket.

The character wu (無) originally meant "dance" and was later used as a graphic loan for wu, "not". The earliest graphs for 無 pictured a person with outstretched arms holding something (possibly sleeves, tassels, ornaments) and represented the word wu "dance; dancer". After wu meaning "dance" was borrowed as a loan for wu meaning "not; without", the original meaning was elucidated with the radical 舛, "opposite feet" at the bottom of wu, 舞 "dance".

The Gateless Gate, a 13th-century collection of Zen kōan, uses the word wu or mu in its title (Wumenguan or Mumonkan 無門關) and first kōan case ("Zhao Zhou's Dog" 趙州狗子). Chinese Chan calls the word mu 無 "the gate to enlightenment".^[10] The Japanese Rinzai school classifies the Mu Kōan as hosshin 発心 "resolve to attain enlightenment", that is, appropriate for beginners seeking kenshō "to see the Buddha-nature"'.^[11]

Case 1 of The Gateless Gate reads as follows:

The koan originally comes from the Zhaozhou Zhenji Chanshi Yulu (Chinese: 趙州真際禪師語錄), The Recorded Sayings of Zen Master Zhao Zhou, koan 132:

The Book of Serenity Chinese: 從容録; pinyin: cóngrónglù, also known as the Book of Equanimity or more formally the Hóngzhì Chánshī Guǎnglù Chinese: 宏智禪師廣錄, has a longer version of this koan, which adds the following to the start of the version given in the Zhaozhou Zhenji Chanshi Yulu.

In Robert M. Pirsig's 1974 novel Zen and the Art of Motorcycle Maintenance, mu is translated as "no thing", saying that it meant "unask the question". He offered the example of a computer circuit using the binary numeral system, in effect using mu to represent high impedance:

For example, it's stated over and over again that computer circuits exhibit only two states, a voltage for "one" and a voltage for "zero." That's silly! Any computer-electronics technician knows otherwise. Try to find a voltage representing one or zero when the power is off! The circuits are in a mu state.^[22]

The word features prominently with a similar meaning in Douglas Hofstadter's 1979 book, Gödel, Escher, Bach. It is used fancifully in discussions of symbolic logic, particularly Gödel's incompleteness theorems, to indicate a question whose "answer" is to either un-ask the question, indicate the question is fundamentally flawed, or reject the premise that a dualistic answer can be given.^[23]

"Mu" may be used similarly to "N/A" or "not applicable," a term often used to indicate that the question cannot be answered because the conditions of the question do not match the reality. An example of this concept could be with the loaded question "Have you stopped beating your wife?", where "mu" would be considered the only respectable response.^[23][24]

The programming language Raku uses "Mu" for the root of its type hierarchy.^[25]

• Wuji (philosophy)
• Wu (awareness) - Chinese concept of enlightenment
• Ma (negative space)
• Wu wei, a term in Chinese philosophy
• Mushin (mental state) - Japanese concept of "no mind"
• Neti neti
• Many-valued logic
• Not even wrong
• Nothing
• Wronger than wrong
• Falsum