Definition Meningitis: an inflammation of the arachnoid membrane the Normal meninges pia mater, and the intervening dura mater arachnoid cerebrospinal fluid(CSF) a mater The inflammatory process extends throughout the ningi subarachnoid space around the brain and spinal cord and involves the ventriclesDefinition Meningitis: an inflammation of the arachnoid membrane, the pia mater, and the intervening cerebrospinal fluid (CSF). The inflammatory process extends throughout the subarachnoid space around the brain and spinal cord and involves the ventricles