Epilepsy is a medical condition that produces seizures affecting a variety of mental and physical functions. It's also called a seizure disorder. A seizure happens when a brief, strong surge of electrical activity affects part or all of the brain. When a person has two or more seizures, they are considered to have epilepsy. For 70 percent of people with seizures and epilepsy, the cause of their condition is unknown. A seizure occurring without a clear cause is called idiopathic.