A febrile seizure, also known as a fever fit or febrile convulsion is a generalized convulsion caused by elevated body temperature. They most commonly occur in children below the age of three years old and should not be diagnosed in children under the age of 6 months or over the age of 6 years. In many cases, the first sign of fever is the onset of the seizure. It has been theorized that the seizure is triggered by the rapidity of the rise in temperature, rather than the actual temperature reached.

Febrile seizures represent the meeting point between a low seizure threshold (genetically and age determined) - some children have a greater tendency to have a seizure under certain circumstances - and a trigger: fever. The genetic causes of febrile seizures are still being researched. Some mutations that cause a neuronal hyperexcitability and could be responsible for febrile seizures have already been discovered.

The diagnosis is one that must be arrived at by eliminating more serious causes of seizure: in particular, meningitis and encephalitis must be ruled out. Therefore a doctor's opinion should be sought and in many cases the child would be admitted to hospital overnight for observation and/or tests. As a general rule, if the child returns to a normal state of health soon after the seizure, a nervous system infection is unlikely. Even in cases where the diagnosis is febrile seizure, doctors will try to identify and treat the source of fever. In particular, it is useful to distinguish the event as a simple febrile seizure - in which the seizure lasts less than 15 minutes, does not recur in the next 24 hours, and involves the entire body. The complex febrile seizure is characterized by long duration, recurrence, or focus on only part of the body. The simple seizure represents the majority of cases and is considered to be less of a cause for concern than the complex. It is reassuring if the cause of seizure can indeed be determined to have been fever, as simple febrile seizures generally do not cause permanent brain injury; do not tend to recur frequently, as children tend to 'out-grow' them; and do not make the development of adult epilepsy significantly more likely.