Dilate means to become wider or more open, or to make something wider or more open. The word is most commonly used to describe what happens to parts of the body. When you walk from bright sunlight into a dark room, the pupils in your eyes dilate (get bigger) to let in more light so you can see better. When you step back into the sunshine, your pupils quickly shrink again.

Doctors use dilating eye drops during eye exams to make your pupils stay wide open so they can look inside and check for problems. The drops can make everything look blurry and bright for a while, but they wear off after a few hours.

The word can also describe other openings that get wider. Blood vessels dilate when you exercise, allowing more blood to flow to your muscles. When someone is very surprised or scared, their pupils might dilate and look larger.

In a more general sense, dilate can mean to speak or write at length about something, though this usage is less common. If someone dilates on a topic, they expand on it in great detail.