Occurs in response to a decrease in light or certain drugs

• Reflex pupillary dilation as a muscle pulls the iris outward

Wikipedia Word definitions in Wikipedia
Mydriasis is the dilation of the pupil , usually defined as when having a non-physiological cause,Mydriasis in Farlex medical dictionary. In turn citing: The American Heritage Medical Dictionary (2007) Mosby's Dental Dictionary, 2nd edition. but sometimes ...

WordNet Word definitions in WordNet
n. reflex pupillary dilation as a muscle pulls the iris outward; occurs in response to a decrease in light or certain drugs

Wiktionary Word definitions in Wiktionary
n. (context medicine English) The condition of having abnormally large and dilated pupils due to disease or drugs, particularly stimulants such as (meth)amphetamines, cocaine, etc.

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Mydriasis \My*dri"a*sis\, n. [L., fr. Gr. ?.] (Physiol. & Med.) A long-continued or excessive dilatation of the pupil of the eye.