A Subclinical seizure is a type of seizure

Seizure

An epileptic seizure, occasionally referred to as a fit, is defined as a transient symptom of "abnormal excessive or synchronous neuronal activity in the brain". The outward effect can be as dramatic as a wild thrashing movement or as mild as a brief loss of awareness...

 often experienced by people with epilepsy, in which an EEG trace will show abnormal brain activity, usually for a short duration of time but which does not present any noticeable clinical signs or symptoms. This is often manifest as a single spike on the EEG trace or a slowing of brain activity not correlating to their level of consciousness or awareness. Subclinical seizures can be useful to a neurologist in the diagnosis of epilepsy.