Epilepsy is a group of neurological disorders characterized by recurrent epileptic seizures.  Epileptic seizures are episodes that can vary from brief and nearly undetectable periods to long periods of vigorous shaking. These episodes can result in physical injuries, including occasionally broken bones.  In epilepsy, seizures have a tendency to recur and, as a rule, have no immediate underlying cause. Isolated seizures that are provoked by a specific cause such as poisoning are not deemed to represent epilepsy. People with epilepsy may be treated differently in various areas of the world and experience varying degrees of social stigma due to their condition.

The underlying mechanism of epileptic seizures is excessive and abnormal neuronal activity in the cortex of the brain. The reason this occurs in most cases of epilepsy is unknown. Some cases occur as the result of brain injury, stroke, brain tumors, infections of the brain, or birth defects through a process known as epileptogenesis.  Known genetic mutations are directly linked to a small proportion of cases. The diagnosis involves ruling out other conditions that might cause similar symptoms, such as fainting, and determining if another cause of seizures is present, such as alcohol withdrawal or electrolyte problems. This may be partly done by imaging the brain and performing blood tests  Epilepsy can often be confirmed with an electroencephalogram (EEG), but a normal test does not rule out the condition.

Epilepsy that occurs as a result of other issues may be preventable. Seizures are controllable with medication in about 70% of cases. inexpensive anti-seizure medications are often available. In those whose seizures do not respond to medication, surgery, neurostimulation or dietary changes may then be considered.  Not all cases of epilepsy are lifelong, and many people improve to the point that treatment is no longer needed.

As of 2015, about 39 million people have epilepsy.  Nearly 80% of cases occur in the developing world.  In 2015, it resulted in 125,000 deaths, an increase from 112,000 in 1990. Epilepsy is more common in older people.  In the developed world, onset of new cases occurs most frequently in babies and the elderly. In the developing world, onset is more common in older children and young adults due to differences in the frequency of the underlying causes. About 5–10% of people will have an unprovoked seizure by the age of 80, and the chance of experiencing a second seizure is between 40 and 50%. In many areas of the world, those with epilepsy either have restrictions placed on their ability to drive or are not permitted to drive until they are free of seizures for a specific length of time. The word epilepsy is from Ancient Greek  'to seize, possess, or afflict'.