| vivarium | (noun) an indoor enclosure for keeping and raising living animals and plants and observing them under natural conditions | - |
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| ytterbium | (noun) a soft silvery metallic element; a rare earth of the lanthanide series; it occurs in gadolinite and monazite and xenotime | Synonyms: atomic number 70, Yb |
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| yttrium | (noun) a silvery metallic element that is common in rare-earth minerals; used in magnesium and aluminum alloys | Synonyms: atomic number 39, Y |
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| zirconium | (noun) a lustrous grey strong metallic element resembling titanium; it is used in nuclear reactors as a neutron absorber; it occurs in baddeleyite but is obtained chiefly from zircon | Synonyms: atomic number 40, Zr |
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| zoarium | (noun) a colony of zooids. | - |
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