1. Proliferation of neuroglia in the brain or spinal cord, as a replacement process or a reaction to low-grade inflammation and which may be diffuse or focal.
2. A process leading to scars in the central nervous system that involves the production of a dense fibrous network of glial cells (supporting cells) in areas of damage.

Gliosis is a prominent feature of many diseases of the central nervous system, including multiple sclerosis and stroke. After a stroke, nerve cells die and are replaced by areas of gliosis