Epilepsy simply refers to repeated seizures. Seizures may occur as a one time event in an animal from a variety of causes, but only if the seizures repeat again and again over a period of time do we call it epilepsy. Seizures are a sign of brain disease the same way a cough is a sign of lung disease. Saying an animal has epilepsy is like saying it has a chronic cough; it is a sign of a problem which isn't going away. Anything which damages the brain in the right area can cause epilepsy. If we can identify the cause of the seizures, say a brain tumor or a stroke, then we say the pet has symptomatic (or secondary) epilepsy. That is, the seizures are a symptom of a disease process we've been able to identify. If we've looked and can't find the cause, then we call it idiopathic (or primary) epilepsy. The termidiopathic simply means that we don't know the cause. It may be that the cause has escaped our attention; for example, a stroke that is too small to detect with routine brain scans or damage that occurred during whelping.
Many of the idiopathic epileptics have inherited epilepsy: epilepsy caused by a mutation in a specific gene which they inherited from their parents. Dogs with idiopathic epilepsy frequently begin seizing at between one and three years of age, and certain breeds are predisposed to develop epilepsy. A few breeds have proven hereditary epilepsy, while in most it is just a strong suspicion.