Final answer:
Mydriasis refers to the dilation or expansion of the pupil. It is triggered by the sympathetic nervous system and can be artificially induced or reversed with medications for various eye treatments.
Step-by-step explanation:
The term that best means dilation or expansion of the pupil is Mydriasis. This dilation is driven by the sympathetic nervous system when norepinephrine binds to an adrenergic receptor in the radial fibers of the iris smooth muscle. Substances like phenylephrine are used to mimic this physiological mydriasis for medical purposes such as eye exams.
Contrarily, pilocarpine is a muscarinic agonist that can reverse mydriasis and is often used after eye examinations or to treat certain eye conditions. Understanding the autonomic control of pupillary size helps explain why pupils dilate in the dark to allow more light and constrict in bright light to limit the amount of light entering the eye.