Arachnidan is a term used to describe spiders and other arachnids. There are several synonyms for this word. For example, arachnoid, arachnid, and araneiform are all synonyms for arachnidan. Arachniform and arachnoidean are also synonyms for this term. These words all describe creatures with eight legs and two body parts. While spiders are the most well-known arachnids, scorpions, ticks, and mites are also included in this group. Arachnids can be found in a variety of habitats, from deserts to forests, and they play an important role in the ecosystem as predators of insects and other small animals.