A seizure is a single event. It is abnormal and excessive discharge of electrical activity from a collection of neurons. This discharge is accompanied by some behavioral change in the individual.
Changes may be dramatic, or may be quite subtle, some are detectable only through specialized tests, such as a precise measurements of reaction time.
Epilepsy is the disorder experienced by people who have multiple seizures, it is a chronic condition characterized by a predisposition to recurrent, usually spontaneous seizures.