any of an order (Araneae syn. Araneida) of arachnids having the abdomen usually unsegmented and constricted at the base, chelicerae modified into poison fangs, and two or more pairs of abdominal spinnerets for spinning threads of silk for various uses (as in making cocoons for their eggs or webs to catch prey)
*This definition was obtained at http://www.merriam-webster.com/dictionary/spider 22 May 2011.