Etymology
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zodiac (n.)
late 14c., from Old French zodiaque, from Latin zodiacus "zodiac," from Greek zodiakos (kyklos) "zodiac (circle)," literally "circle of little animals," from zodiaion, diminutive of zoion "animal" (from PIE root *gwei- "to live").

Libra is not an animal, but it was not a zodiac constellation to the Greeks, who reckoned 11 but counted Scorpio and its claws (including what is now Libra) as a "double constellation." Libra was figured back in by the Romans. In Old English the zodiac was twelf tacna "the twelve signs," and in Middle English also Our Ladye's Waye and the Girdle of the Sky.
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*gwei- 
also *gweie-, Proto-Indo-European root meaning "to live."

It forms all or part of: abiogenesis; aerobic; amphibian; anaerobic; azo-; azoic; azotemia; bio-; biography; biology; biome; bionics; biopsy; biota; biotic; cenobite; Cenozoic; convivial; couch-grass; epizoic; epizoon; epizootic; macrobiotic; Mesozoic; microbe; Protozoa; protozoic; quick; quicken; quicksand; quicksilver; quiver (v.) "to tremble;" revive; survive; symbiosis; viable; viand; viper; vita; vital; vitamin; victuals; viva; vivace; vivacious; vivarium; vivid; vivify; viviparous; vivisection; whiskey; wyvern; zodiac; Zoe; zoetrope; zoic; zoo-; zoolatry; zoology; zoon; zoophilia; zoophobia; zooplankton.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit jivah "alive, living;" Old Persian *jivaka- "alive," Middle Persian zhiwak "alive;" Greek bios "one's life, course or way of living, lifetime," zoe "animal life, organic life;" Old English cwic, cwicu "living, alive;" Latin vivus "living, alive," vita "life;" Old Church Slavonic zivo "to live;" Lithuanian gyvas "living, alive," gyvata "(eternal) life;" Old Irish bethu "life," bith "age;" Welsh byd "world."
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cancer (n.)
Old English cancer "spreading sore, malignant tumor" (also canceradl), from Latin cancer "a crab," later, "malignant tumor," from Greek karkinos, which, like the Modern English word, has three meanings: a crab, a tumor, and the zodiac constellation represented by a crab, from PIE *karkro-, reduplicated form of root *kar- "hard."

Greek physicians Hippocrates and Galen, among others, noted similarity of crabs to some tumors with swollen veins. The Old English word was displaced by French-influenced doublet canker but was reintroduced in the modern medical sense c. 1600. In reference to the zodiac sign from late Old English; meaning "person born under the zodiac sign of Cancer" is from 1894. The sun being in Cancer at the summer solstice, the constellation had association in Latin writers with the south and with summer heat. Cancer stick "cigarette" is from 1959.
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Scorpio (n.)

zodiacal constellation, late 14c., from Latin scorpio (poetic scorpius) "scorpion," also the zodiac constellation (see scorpion). The meaning "person born under or ruled by the sign of Scorpio" is recorded from 1941. Since 1922 the official (International Astronomical Union) scientific designation of the constellation has been Scorpius, leaving Scorpio to astrology.

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decile (adj.)

1670s in astrology, of planets, "one-tenth part of the zodiac distant from one another;" 1882 in statistics; from French décile or Medieval Latin *decilis, from Latin decem "ten" (from PIE root *dekm- "ten") on the model of quintilis, sextilis.

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Pisces (n.)

ancient constellation, 12th sign of the zodiac, representing two fishes united by a cord attached to their tails, late Old English, from Latin pisces, plural of piscis "a fish" (from PIE root *pisk- "a fish"). Applied to persons born under this sign by 1924 (also Piscean).

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Capricorn 

zodiac sign represented as a goat, or half-goat half-fish, late Old English, from Latin Capricornus, literally "horned like a goat," from caper (genitive capri) "goat" (see cab) + cornu "horn" (from PIE root *ker- (1) "horn; head"). A loan-translation of Greek Aigokherōs, the name of the constellation. Extended 1894 to persons born under the sign.

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quintile (n.)

1610s, originally in astrology and said to have been introduced by Kepler, "aspect of planets when they are 72 degrees from each other" (a fifth of the zodiac), from Latin quintus "the fifth" (from PIE root *penkwe- "five") + ending from quartile. Its use in statistics in reference to a division data points into five parts of more or less equal size dates to 1951.

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Leo 
zodiac constellation, late Old English, from Latin leo "lion" (see lion). Meaning "person born under the sign of Leo" is from 1894. Leonid "meteor which appears to radiate from Leo" is from 1868; the annual shower peaks Nov. 14 and the stars fall in extreme profusion about every 33 years. The meteors are believed now to be associated with comet Tempel–Tuttle. The dim constellation Leo Minor was introduced 1690 by Hevelius.
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