Inflammation of the arachnoid membrane: Arachnoiditis can be any process of acute, subacute, or chronic inflammation, scarring, proliferative orgranulomatous process in the arachnoid membrane, whether primary or secondary, whether isolated or associated with damage to contiguous nervous tissues.

Types of arachnoiditis, designated according to their situation, include optochiasmatic, basal, spinal, and many other forms.