I did some research...
"Located at the back of the midbrain, these are usually solid, focal, slow-growing tumors with a gradual onset of symptoms" is the definition of a tectal glioma.
A glioma itself is:
"Glioma” is a general term used to describe any tumor that arises from the supportive (“gluey”) tissue of the brain. This tissue, called “glia,” helps to keep the neurons in place and functioning well.