What condition is defined as a seizure disorder caused by disturbed nerve cell activity in the brain?

Epilepsy is a condition characterized by recurrent seizures, which occur due to abnormal and excessive neuronal activity in the brain. This disturbance in nerve cell activity can lead to a variety of symptoms depending on the part of the brain involved and the nature of the seizure. These seizures may manifest in numerous ways, including convulsions, loss of consciousness, or altered sensations.

In contrast, other conditions mentioned do not primarily involve seizure activity as a defining symptom. While multiple sclerosis can lead to a variety of neurological symptoms, it is primarily an autoimmune disorder affecting the central nervous system rather than a seizure disorder. Parkinson's disease is primarily known for its motor symptoms such as tremors and rigidity, rather than seizures. A brain tumor can cause seizures, but it is not categorized as a seizure disorder itself; it represents a structural abnormality in the brain that may lead to various symptoms, including seizures, depending on its location and size. Thus, epilepsy stands out as the correct condition due to its direct association with seizure activity resulting from disrupted nerve cell functioning.