Definition: Araneoidea is a family of spiders and arachnids that include many species in the order Araneae, which are commonly known as arachnids. Detailed definition: Araneoidea includes more than 20,000 species, ranging from small, microscopic arachnids found on plants to larger, large-bodied spiders found on land. Araneas and arachnids are members of the arthropod family.