A client reports sensing an unusual smell just prior to experiencing a tonic-clonic seizure. what term is used to describe this event?

This would be called an aura. An aura is simply some sort of disturbance that is experienced by people who have migraines or seizures. It is something they sense, see or smell prior to the headache or seizure beginning. Sometimes it is a strange light or a foul odor, sometimes it can even be confusing thoughts. Generally, those who experience auras will have the same aura each time.