Hundun (Chinese: 混沌; pinyin: Hùndùn; Wade–Giles: Hun⁴-tun⁴; lit. 'muddled confusion') is both a "legendary faceless being" in Chinese mythology and the "primordial and central chaos" in Chinese cosmogony, comparable with the world egg.

Hundun 混沌 was semantically extended from a mythic "primordial chaos; nebulous state of the universe before heaven and earth separated" to mean "unintelligible; chaotic; messy; mentally dense; innocent as a child".

While hùndùn "primordial chaos" is usually written as 混沌 in contemporary vernacular, it is also written as 渾沌—as in the Daoist classic Zhuangzi—or 渾敦 —as in the Zuozhuan. Hùn "chaos; muddled; confused" is written either hùn (混; 'abundantly flowing', 'turbid water', 'torrent', 'mix up/in', 'confuse', 'thoughtless', 'senseless') or hún (渾; 'sound of running water', 'muddy', 'muddled', 'confused', 'dull', 'stupid'). These two are interchangeable graphic variants read as hún (混; 'muddy', 'dirty', 'filthy') ) and hùn (渾; 'nebulous', 'stupid') (渾沌; hùndùn). Dùn ("dull; confused") is written as either dùn (沌; 'dull', 'confused', 'stupid') or dūn (敦; 'thick', 'solid', 'generous', 'earnest', 'honest', 'sincere').

Isabelle Robinet outlines the etymological origins of hundun.

Semantically, the term hundun is related to several expressions, hardly translatable in Western languages, that indicate the void or a barren and primal immensity – for instance, hunlun (混淪), hundong (混洞), kongdong (空洞), menghong (蒙洪), or hongyuan (洪元). It is also akin to the expression "something confused and yet complete" (混成; huncheng) found in the Daode jing 25, which denotes the state prior to the formation of the world where nothing is perceptible, but which nevertheless contains a cosmic seed. Similarly, the state of hundun is likened to an egg; in this usage, the term alludes to a complete world round and closed in itself, which is a receptacle like a cavern (洞; dong) or a gourd (壺; hu or 壺盧; hulu).

Most Chinese characters are written using "radicals" or "semantic elements" and "phonetic elements". Hùndùn (混沌) is written with the "water radical" 水 or 氵 and phonetics of kūn (昆) and tún (屯). Hùndùn "primordial chaos" is cognate with Wonton (餛飩; húntun) "wonton; dumpling soup" written with the "eat radical" 食. Note that the English loanword wonton is borrowed from the Cantonese pronunciation wan⁴tan¹. Victor H. Mair suggests a fundamental connection between hundun and wonton: "The undifferentiated soup of primordial chaos. As it begins to differentiate, dumpling-blobs of matter coalesce. … With the evolution of human consciousness and reflectiveness, the soup was adopted as a suitable metaphor for chaos". This last assertion appears unsupported, however, since wonton soup is not attested in Chinese sources dating earlier than the Han dynasty, although the linguistic connection of the soup to the larger concept certainly appears real.

Hundun 混沌 has a graphic variant 混淪; hunlun (using 淪; lún; 'ripples', 'eddying water', 'sink down' see the Liezi below), which etymologically connects to the mountain name Kunlun 崑崙 (differentiated with the "mountain radical" 山). Robinet says "Kunlun and hundun are the same closed center of the world."

Girardot quotes the Chinese philologist Lo Mengci 羅夢冊, who says that reduplicated words like hundun "suggest cyclic movement and transformation", and speculates: