Ask FunTrivia strives to be the most accurate and complete source for answers to trivia questions on the web. Please cite all facts with citation links or references from trustable sources. Editors continuously recheck submissions and claims.
Definition of arachnoid. 1  1 : of or relating to a thin membrane of the brain and spinal cord that lies between the dura mater and the pia mater. 2  2 : covered with or composed of soft loose hairs or fibers.
Arachnoid cysts are fluid-filled sacs that develop between the central nervous system (brain and spinal cord) and the arachnoid membrane, one of three protective membranes that surround the central nervous system.