Medical Definition of spider
1: any of an order (Araneae synonym Araneida) of arachnids having a body with two main divisions, four pairs of walking legs, and two or more pairs of abdominal spinnerets for spinning threads of silk used especially in making webs for catching prey
2: spider nevus <an arterial spider>
3: an obstruction in the teat of a cow; especially : a small irregular horny growth resulting from irritation or bruising
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