These Chinese terms yin 陰 or 阴 "shady side" and yang 陽 or 阳 "sunny side" are linguistically analyzable in terms of Chinese characters, pronunciations and etymology, meanings, topography, and loanwords.
The Traditional Chinese characters 陰 and 陽 for the words yīn and yáng are both classified as radical-phonetic characters, combining the semantically significant "mound; hill" radical 阝 or 阜 with the phonetic indicators ying 侌 and yang 昜. The first phonetic yīn 侌 "cloudy" ideographically combines jīn 今 "now; present" and yún 云 "cloud", denoting the "今 presence of 云 clouds". The second phonetic yáng 昜 "bright" originally pictured 日 the "sun" with 勿 "rays coming down". This phonetic is expanded with the "sun" radical into yáng 暘 "rising sun; sunshine". The "mound; hill" radical 阝full forms semantically specify yīn 陰 "shady/dark side of a hill" and yáng 陽 "sunny/light side of a hill".
The Simplified Chinese characters 阴 and 阳 for yīn and yáng combine the same "hill" radical 阝 with the non-phonetic yuè 月 "moon" and rì 日 "sun", graphically denoting "shady side of a hill" and "sunny side of a hill". Compare the Classical Chinese names (which contain tài 太 "great") for these two heavenly bodies: Tàiyīn 太陰 "moon" and Tàiyáng 太陽 "sun".
Pronunciations and etymologies
The Modern Standard Chinese pronunciation of 陰 or 阴 is usually level first tone yīn "shady; cloudy" or sometimes falling fourth tone yìn "to shelter; shade", and 陽 or 阳 "sunny" is always pronounced with rising second tone yáng.
Sinologists and historical linguists have reconstructed Middle Chinese pronunciations from data in the (7th century CE) Qieyun rhyme dictionary and later rhyme tables, which was subsequently used to reconstruct Old Chinese phonology from rhymes in the (11th-7th centuries BCE) Shijing and phonological components of Chinese characters. Reconstructions of Old Chinese have illuminated the etymology of modern Chinese words.
Compare these Middle Chinese and Old Chinese (with asterisk) reconstructions of yīn 陰 and yáng 陽:
Schuessler gives probable Sino-Tibetan etymologies for both Chinese words.
Yin < *ʔəm compares with Burmese ʔumC "overcast; cloudy", Adi muk-jum "shade", and Lepcha so'yǔm "shade"; and is probably cognate with Chinese àn < *ʔə̂mʔ 黯 "dim; gloomy" and qīn < *khəm 衾 "blanket".
Yang < *laŋ compares with Lepcha a-lóŋ "reflecting light", Burmese laŋB "be bright" and ə-laŋB "light"; and is perhaps cognate with Chinese chāng < *k-hlaŋ 昌 "prosperous; bright" (compare areal words like Tai plaŋA1 "bright" & Proto-Viet-Muong hlaŋB). To this word-family, Unger (Hao-ku, 1986:34) also includes 炳 bǐng < *pl(j)aŋʔ "bright"; however Schuessler reconstructs 炳's Old Chinese pronunciation as *braŋʔ and includes it in an Austroasiatic word family, besides 亮 liàng < *raŋh 爽 shuǎng < *sraŋʔ "twilight (of dawn)"; míng < mraŋ 明 "bright, become light, enlighten"; owing to "the different OC initial consonant which seems to have no recognizable OC morphological function".
Yin and yang are semantically complex words.
John DeFrancis's Chinese-English dictionary gives the following translation equivalents.
Yin 陰 or 阴 Noun ① [philosophy] negative/passive/female principle in nature ② Surname Bound morpheme ① the moon ② shaded orientation ③ covert; concealed; hidden ④ ⑦ negative ⑧ north side of a hill ⑨ south bank of a river ⑩ reverse side of a stele ⑪in intaglio Stative verb ① overcast
Yang 陽 or 阳 Bound morpheme ① [Chinese philosophy] positive/active/male principle in nature ②the sun ④ in relief ⑤ open; overt ⑥ belonging to this world ⑦ [linguistics] masculine ⑧ south side of a hill ⑨ north bank of a river
The compound yinyang 陰陽 or 阴阳 means "yin and yang; opposites; ancient Chinese astronomy; occult arts; astrologer; geomancer; etc.".
The sinologist Rolf Stein etymologically translates Chinese yin 陰 "shady side (of a mountain)" and yang 陽 "sunny side (of a mountain)" with the uncommon English geographic terms ubac "shady side of a mountain" and adret "sunny side of a mountain" (which are of French origin).
Many Chinese place names or toponyms contain the word yang "sunny side" and a few contain yin "shady side". In China, as elsewhere in the Northern Hemisphere, sunlight comes predominantly from the south, and thus the south face of a mountain or the north bank of a river will receive more direct sunlight than the opposite side.
Yang refers to the "south side of a hill" in Hengyang 衡陽, which is south of Mount Heng 衡山 in Hunan province, and to the "north bank of a river" in Luoyang 洛陽, which is located north of the Luo River 洛河 in Henan.
Similarly, yin refers to "north side of a hill" in Huayin 華陰, which is north of Mount Hua 華山 in Shaanxi province.
In Japan, the characters are used in western Honshu to delineate the north-side San'in region 山陰 from the south-side San'yō region 山陽, separated by the Chūgoku Mountains 中国山地.
English yin, yang, and yin-yang are familiar loanwords of Chinese origin.
The Oxford English Dictionary defines:
yin (jɪn) Also Yin, Yn. [Chinese yīn shade, feminine; the moon.]
a. In Chinese philosophy, the feminine or negative principle (characterized by dark, wetness, cold, passivity, disintegration, etc.) of the two opposing cosmic forces into which creative energy divides and whose fusion in physical matter brings the phenomenal world into being. Also attrib. or as adj., and transf. Cf. yang.
b. Comb., as yin-yang, the combination or fusion of the two cosmic forces; freq. attrib., esp. as yin-yang symbol, a circle divided by an S-shaped line into a dark and a light segment, representing respectively yin and yang, each containing a 'seed' of the other.
yang (jæŋ) Also Yang. [Chinese yáng yang, sun, positive, male genitals.]
a. In Chinese philosophy, the masculine or positive principle (characterized by light, warmth, dryness, activity, etc.) of the two opposing cosmic forces into which creative energy divides and whose fusion in physical matter brings the phenomenal world into being. Also attrib. or as adj. Cf. yin.
b. Comb.: yang-yin = yin-yang s.v. yin b.
For the earliest recorded "yin and yang" usages, the OED cites 1671 for yin and yang, 1850 for yin-yang, and 1959 for yang-yin.
In English, yang-yin (like ying-yang) occasionally occurs as a mistake or typographical error for the Chinese loanword yin-yang— yet they are not equivalents. Chinese does have some yangyin collocations, such as 洋銀 (lit. "foreign silver") "silver coin/dollar", but not even the most comprehensive dictionaries (e.g., the Hanyu Da Cidian) enter yangyin *陽陰. While yang and yin can occur together in context, yangyin is not synonymous with yinyang. The linguistic term "irreversible binomial" refers to a collocation of two words A-B that cannot be idiomatically reversed as B-A, for example, English cat and mouse (not *mouse and cat) and friend or foe (not *foe or friend).
Similarly, the usual pattern among Chinese binomial compounds is for positive A and negative B, where the A word is dominant or privileged over B, for example, tiandi 天地 "heaven and earth" and nannü 男女 "men and women". Yinyang meaning "dark and light; female and male; moon and sun", however, is an exception. Scholars have proposed various explanations for why yinyang violates this pattern, including "linguistic convenience" (it is easier to say yinyang than yangyin), the idea that "proto-Chinese society was matriarchal", or perhaps, since yinyang first became prominent during the late Warring States period, this term was "purposely directed at challenging persistent cultural assumptions".