Functional hemispherectomy: A functional hemispherectomy is a variation of a hemispherectomy, a radical procedure in which one entire hemisphere, or one half of the brain, is removed. With a functional hemispherectomy, one hemisphere is disconnected from the rest of the brain, but only a limited area of brain tissue is removed. This surgery generally is limited to children younger than 13 years old who have one hemisphere that is not functioning normally.