Physiological anisocoria: This condition is characterized by a normal, benign variation in pupil size between the two eyes, often noticed under certain lighting conditions. It's the most common cause of a dilated pupil and is usually asymptomatic.

Oculomotor nerve (CN III) palsy: Damage to the third cranial nerve can cause a dilated pupil due to paralysis of the iris sphincter muscle. This can result from various causes such as diabetes, aneurysms, or trauma.
Local eye conditions: Conditions like iritis, uveitis, or a posterior synechiae can cause a dilated pupil due to inflammation or adhesions within the eye.
Trauma: Blunt or penetrating trauma to the eye can cause a dilated pupil due to injury to the iris or the nerves controlling pupil size.

Mydriatic drug exposure: Accidental or intentional exposure to certain medications (e.g., atropine, scopolamine) can cause unilateral or bilateral pupil dilation.
Herniation syndrome: Increased intracranial pressure causing transtentorial herniation can compress the oculomotor nerve, leading to a dilated pupil on the same side as the herniation. This is a medical emergency.
Carotid artery dissection or aneurysm: Compression or damage to the oculomotor nerve by a carotid artery dissection or aneurysm can cause a dilated pupil.

Adie's pupil (Tonic pupil): A rare condition characterized by a dilated pupil that reacts poorly to light but has a more pronounced response to near vision. It's often associated with diminished or absent deep tendon reflexes.
Sympathetic nervous system disorders: Conditions affecting the sympathetic nervous system, such as Horner's syndrome, can cause a constricted pupil on the affected side, but in rare cases, can present with anisocoria due to disruption of normal sympathetic input to the eye.