any wingless, carnivorous arthropod of the class Arachnida, including spiders, scorpions, mites, ticks, and daddy-longlegs, having a body divided into two parts, the cephalothorax and the abdomen, and having eight appendages and no antennae. Compare insect.
belonging or pertaining to the arachnids.

1865–70; < Neo-Latin Arachnida < Greek aráchn(ē) spider, spider's web + Neo-Latin -ida -ida

arachnidan [uh-rak-ni-duhn] , adjective, noun Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
arachnid (əˈræknɪd)
any terrestrial chelicerate arthropod of the class Arachnida, characterized by simple eyes and four pairs of legs. The group includes the spiders, scorpions, ticks, mites, and harvestmen
[C19: from New Latin Arachnida, from Greek arakhnē spider]
adj, —n

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

1869, from Fr. arachnide (1806) or Mod.L. Arachnida, introduced as name for this class of arthropods 1815 by Fr. biologist Jean Baptiste Pierre Antoine de Monet de Lamarck (1744-1829), from Gk. arakhne "spider," which probably is cognate with L. aranea "spider, spider's web" (from aracsna).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
arachnid   (ə-rāk'nĭd)  Pronunciation Key 
Any of various arthropods of the class Arachnida, such as spiders, scorpions, mites, and ticks. Arthropods are characterized by four pairs of segmented legs and a body that is divided into two regions, the cephalothorax and the abdomen.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
The nocturnal arachnid uses venom to subdue their prey.
Among arachnid enthusiasts, these spiders have become popular pets.
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