a·rach·noid 
(ə-răknoid′)
(ə-răknoid′)adj.
1. Anatomy Of or relating to a delicate membrane enclosing the spinal cord and brain.
2. Of, relating to, or resembling arachnids.
3. Covered with or consisting of thin, soft, entangled hairs or fibers resembling those of a cobweb or spider's web.
n.
1. An arachnid.
2. Anatomy A delicate membrane that encloses the spinal cord and brain and lies between the pia mater and dura mater.
[New Latin arachnoīdēs, from Greek arakhnoeidēs, cobweblike : arakhnē, spider + -oeidēs, -oid.]