The Collaborative International Dictionary of English v.0.48:

Spider web \Spi"der web"\, or Spider's web \Spi"der's web"\ .
   (Zool.)
   The silken web which is formed by most kinds of spiders,
   particularly the web spun to entrap their prey; -- also
   called cobweb. See Geometric spider, Triangle spider,
   under Geometric, and Triangle.
   [1913 Webster]

WordNet (r) 3.0 (2006):

spider web
    n 1: a web resembling the webs spun by spiders [syn: spider
         web, spider's web]
    2: a web spun by spiders to trap insect prey [syn: spider web,
       spider's web]