A focal seizure, also known as a partial seizure or localized seizure, is a type of seizure that affects only one hemisphere of the brain. The brain is divided into two hemispheres, each consisting of four lobes - the frontal, temporal, parietal, and occipital lobes. Focal seizures can be simple or complex, and they can be divided into two main categories: focal onset aware and focal onset impaired awareness.

• Focal onset aware seizures: These seizures begin in one side of the brain, and the person remains conscious and aware of their surroundings during the seizure. They were previously called simple partial seizures or auras.

• Focal onset impaired awareness seizures: These seizures affect a larger part of the hemisphere, and the person may lose consciousness. They were previously called complex partial seizures.

Focal seizures can have different symptoms depending on the location in the brain and parts of the body affected. Some things that may happen during a simple focal seizure are numbness or tingling, hearing problems, hallucinations, olfactory or other distortions, and changes in blood pressure, heart rhythm, or bowel or bladder function. During a complex focal seizure, a person may not know what is happening, and they may experience confusion, in a "fighting" mood, or hard to wake up, or they may sleep for several hours after the seizure.

If a focal seizure spreads from one hemisphere to the other side of the brain, this will give rise to a focal to bilateral seizure. The person will become unconscious and may experience a tonic-clonic seizure. Focal seizures can be caused by various conditions such as stroke, head injuries, birth abnormalities of the brain, infections of the brain, brain tumors, or other conditions that affect the brain. Treatment for focal seizures may include anticonvulsant medications, diet therapy, nerve stimulation, or surgery, depending on the seizures characteristics.