Brain lesions are abnormalities seen on imaging tests such as MRIs or CT scans that appear as dark or light spots separate from normal brain tissue. These can be caused by a brain aneurysm, tumor, encephalitis, epilepsy, multiple sclerosis, stroke, or traumatic brain injury.
Brain Lesions. May Clinic, https://www.mayoclinic.org/symptoms/brain-lesions/basics/when-to-see-doctor/sym-20050692.
Brain Tumors in Adults, WebMD, https://www.webmd.com/cancer/brain-cancer/brain-tumors-in-adults#2