An epileptic seizure, informally known as a seizure, is a period of symptoms due to abnormally excessive or synchronous neuronal activity in the brain. Outward effects vary from uncontrolled shaking movements involving much of the body with loss of consciousness (tonic-clonic seizure), to shaking movements involving only part of the body with variable levels of consciousness (focal seizure), to a subtle momentary loss of awareness (absence seizure). Most of the time these episodes last less than two minutes and it takes some time to return to normal. Loss of bladder control may occur.

Seizures may be provoked and unprovoked. Provoked seizures are due to a temporary event such as low blood sugar, alcohol withdrawal, abusing alcohol together with prescription medication, low blood sodium, fever, brain infection, or concussion. Unprovoked seizures occur without a known or fixable cause such that ongoing seizures are likely. Unprovoked seizures may be exacerbated by stress or sleep deprivation. Epilepsy describes brain disease in which there has been at least one unprovoked seizure and where there is a high risk of additional seizures in the future. Conditions that look like epileptic seizures but are not include: fainting, nonepileptic psychogenic seizure and tremor.

A seizure that lasts for more than a brief period is a medical emergency. Any seizure lasting longer than five minutes should be treated as status epilepticus. A first seizure generally does not require long-term treatment with anti-seizure medications unless a specific problem is found on electroencephalogram (EEG) or brain imaging. Typically it is safe to complete the work-up following a single seizure as an outpatient. In many, with what appears to be a first seizure, other minor seizures have previously occurred.

Up to 10% of people have had at least one epileptic seizure in their lifetime. Provoked seizures occur in about 3.5 per 10,000 people a year while unprovoked seizures occur in about 4.2 per 10,000 people a year. After one seizure, the chance of experiencing a second one is about 50%. Epilepsy affects about 1% of the population at any given time