What is the gap between arachnoid and pia mater called?

The cerebrospinal fluid-filled region between the arachnoid and the pia is known as the subarachnoid space. 

The cerebrospinal fluid (CSF), major blood arteries, and cisterns make up the subarachnoid area. 

Because of the structure of the brain and spinal cord surface, the cisterns are larger pockets of CSF generated by the separation of the arachnoid mater from the pia mater.

Hence, the correct option is subarachnoid space.