Epilepsy refers to multiple seizures(more than one), And it’s Seizure when it occurs once only.
A seizure is a fit which is single where as epilepsy is a neurological condition characterized by two or more unprovoked seizures.
To clarify multiple/single: someone can have a seizure and never have one again, but epilepsy patients have reccuring seisures. To be diagnosed with epilepsy, there is a specific timing and criteria that a doctor determines.