Seizures, which range from unusual mood swings to uncontrollable thrashing and loss of body control, are symptoms of a disease. Causes of seizures include infections, tumors, toxic chemicals, and epilepsy. Most seizures in cats have a cause other than epilepsy. It's most useful to think of epilepsy as a word for seizures for which no other cause has been found. Idiopathic epilepsy is the term used for seizures that appear to have no other cause.
Symptomatic epilepsy refers to seizures caused by an underlying condition.
Regardless of the terms used, the primary goal when treating a cat with seizures is to identify the disease causing the seizures, assuming it is epilepsy only if no other cause can be found. Seizures should not go untreated. There are risks of choking during a seizure, and more importantly, the seizures may be symptomatic of an underlying disease that can and should be treated. Anti-convulsant drugs may be effective in controlling the seizures, depending on the exact diagnosis. If you observe your cat having what you believe is a seizure, the most important thing to do is to observe every detail you can about it so it can be described to your vet.