What exactly is a brain hamartoma?

Hamartomas. A hamartoma is a common benign tumor in an organ composed of tissue elements normally found at that site but that are growing in a disorganized mass. They occur in many different parts of the body and are most often asymptomatic and undetected unless seen on an image taken for another reason. Hamartomas result from an abnormal formation of normal tissue, and usually only cause problems by location.
Focal malformation. A focal tissue malformation. Looks like a tumor but is not. May cause seizures at times.