adjective
Definition: Relating to a thin, delicate membrane that surrounds the brain and spinal cord, or resembling arachnids, which are spider-like creatures.
Example: The arachnoid layer protects the brain and spinal cord from injury.
noun
Definition: A thin, delicate membrane that surrounds the brain and spinal cord, located between two other membranes called the pia mater and dura mater.
Example: The doctor explained that the arachnoid is one of the protective layers around the brain.