The Collaborative International Dictionary of English v.0.48:

Arachnoid \A*rach"noid\, a. [Gr. ? like a cobweb; ? spider,
   spider's web + e'i^dos form.]
   1. Resembling a spider's web; cobweblike.
      [1913 Webster]

   2. (Anat.) Pertaining to a thin membrane of the brain and
      spinal cord, between the dura mater and pia mater.
      [1913 Webster]

   3. (Bot.) Covered with, or composed of, soft, loose hairs or
      fibers, so as to resemble a cobweb; cobwebby.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Arachnoid \A*rach"noid\, n.
   1. (Anat.) The arachnoid membrane.
      [1913 Webster]

   2. (Zool.) One of the Arachnoidea.
      [1913 Webster]

WordNet (r) 3.0 (2006):

arachnoid
    adj 1: relating to or resembling a member of the class Arachnida
           [syn: arachnoid, arachnidian, spidery,
           spiderlike, spiderly]
    n 1: the middle of the 3 meninges [syn: arachnoid, arachnoid
         membrane]
    2: air-breathing arthropods characterized by simple eyes and
       four pairs of legs [syn: arachnid, arachnoid]